[LG Optimus G Pro] Floomi:
Final update: I replaced the battery and this issue all but went away. I reported the kernel panic to CyanogenMod and it's tracked in their JIRA as CYAN-6027.

OP saved below for posterity.

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Hey XDA wizards,

Update: This issue is happening while using wifi too, so ignore the reference to "mobile data" in the post title. It's any data access.

My OG Pro (AT&T GSM edition, unlocked, on T-Mobile) has started misbehaving lately. If I'm doing something that requires use of the Internet while my phone is unplugged, it suddenly reboots. The problem is worse if I'm using mobile data rather than wifi, but I've seen it happen using both.

Once this starts happening it won't stop - every time it wants data, it'll reboot or power down. It often won't even make it to the home screen before it dies again. The only way to fix it is by plugging it in to charge, but it's not that the battery is low - it reports well above 80-90%, and will do this immediately after an overnight charge. I can't figure out what's wrong for the life of me - it works fine if it's plugged in.

I was on the stock KitKat ROM but switched to CM11 in the hope that it was a bizarre software issue. I've rebooted into recovery and wiped the cache and the issue still persists.

Any help would be much appreciated!

Update 2: Looks like it's a kernel panic. adb logcat over wifi:

mdm_status_change: unexpected reset external modem
subsys-restart: subsystem_restart_dev(): Restart sequence requested for external_modem, restart_level = 1.
Kernel panic - not syncing: subsys-restart: Resetting the SoC - external_modem crashed.
[<c0014e0c>] (unwind_backtrace+0x0/0x11c) from [<c07d4808>] (panic+0x90/0x1f0)
[<c07d4808>] (panic+0x90/0x1f0) from [<c005e3e4>] (subsystem_restart_dev+0x11c/0x174)
[<c005e3e4>] (subsystem_restart_dev+0x11c/0x174) from [<c008019c>] (mdm_status_change+0x90/0x104)
[<c008019c>] (mdm_status_change+0x90/0x104) from [<c00e266c>] (irq_thread+0x84/0x148)
[<c00e266c>] (irq_thread+0x84/0x148) from [<c00a36c4>] (kthread+0x84/0x94)
[<c00a36c4>] (kthread+0x84/0x94) from [<c000f3d4>] (kernel_thread_exit+0x0/0x8)
CPU2: stopping
[<c0014e0c>] (unwind_backtrace+0x0/0x11c) from [<c0013748>] (handle_IPI+0x100/0x1d4)
[<c0013748>] (handle_IPI+0x100/0x1d4) from [<c0008658>] (gic_handle_irq+0x100/0x10c)
[<c0008658>] (gic_handle_irq+0x100/0x10c) from [<c07e2980>] (__irq_svc+0x40/0x70)
Exception stack(0xd5959e10 to 0xd5959e58)
9e00: 00000000 00000000 00000002 c0f40f80
9e20: 00000282 00000000 fa003000 d5959ec8 0000000a c0832158 3f2f9f3c 00000000
9e40: c0d42c80 d5959e58 c008ce90 c008c83c 20000113 ffffffff
[<c07e2980>] (__irq_svc+0x40/0x70) from [<c008c83c>] (__do_softirq+0x64/0x27c)
[<c008c83c>] (__do_softirq+0x64/0x27c) from [<c008ce90>] (irq_exit+0x48/0xa0)
[<c008ce90>] (irq_exit+0x48/0xa0) from [<c000f184>] (handle_IRQ+0x80/0xc0)
[<c000f184>] (handle_IRQ+0x80/0xc0) from [<c00085e8>] (gic_handle_irq+0x90/0x10c)
[<c00085e8>] (gic_handle_irq+0x90/0x10c) from [<c07e2980>] (__irq_svc+0x40/0x70)
Exception stack(0xd5959ec8 to 0xd5959f10)
9ec0: 00000000 04c704c7 00010000 c0f42f60 60010013 d6ef1500
9ee0: 00000005 c0da7ee0 d5959f88 c0832158 d7c3bd40 00000000 00000010 d5959f10
9f00: c00bd964 c07e256c 60000013 ffffffff
[<c07e2980>] (__irq_svc+0x40/0x70) from [<c07e256c>] (_raw_spin_unlock_irqrestore+0x58/0x88)
[<c07e256c>] (_raw_spin_unlock_irqrestore+0x58/0x88) from [<c00bd964>] (wake_lock_store+0x70/0x90)
[<c00bd964>] (wake_lock_store+0x70/0x90) from [<c026c538>] (kobj_attr_store+0x14/0x20)
[<c026c538>] (kobj_attr_store+0x14/0x20) from [<c01979f4>] (sysfs_write_file+0x100/0x148)
[<c01979f4>] (sysfs_write_file+0x100/0x148) from [<c0147354>] (vfs_write+0xa8/0x180)
[<c0147354>] (vfs_write+0xa8/0x180) from [<c0147644>] (sys_write+0x34/0x68)
[<c0147644>] (sys_write+0x34/0x68) from [<c000e260>] (ret_fast_syscall+0x0/0x30)
CPU3: stopping
[<c0014e0c>] (unwind_backtrace+0x0/0x11c) from [<c0013748>] (handle_IPI+0x100/0x1d4)
[<c0013748>] (handle_IPI+0x100/0x1d4) from [<c0008658>] (gic_handle_irq+0x100/0x10c)
[<c0008658>] (gic_handle_irq+0x100/0x10c) from [<c07e2980>] (__irq_svc+0x40/0x70)
Exception stack(0xd08f9de8 to 0xd08f9e30)
9de0: 00000000 00000000 00000003 c0f40f80 00000202 00000000
9e00: fa003000 d08f9ea0 0000000a d08f8000 3f2f9f3c 00000000 c0d42c80 d08f9e30
9e20: c008ce90 c008c83c 20000113 ffffffff
[<c07e2980>] (__irq_svc+0x40/0x70) from [<c008c83c>] (__do_softirq+0x64/0x27c)
[<c008c83c>] (__do_softirq+0x64/0x27c) from [<c008ce90>] (irq_exit+0x48/0xa0)
[<c008ce90>] (irq_exit+0x48/0xa0) from [<c000f184>] (handle_IRQ+0x80/0xc0)
[<c000f184>] (handle_IRQ+0x80/0xc0) from [<c00085e8>] (gic_handle_irq+0x90/0x10c)
[<c00085e8>] (gic_handle_irq+0x90/0x10c) from [<c07e2980>] (__irq_svc+0x40/0x70)
Exception stack(0xd08f9ea0 to 0xd08f9ee8)
9ea0: d7702e00 00000000 000003b8 00000000 000003b8 00000000 00000003 c0da8ccc
9ec0: d7702e00 d08f8000 00000000 00000000 c00d5a00 d08f9ee8 c00d5a10 c00d553c
9ee0: 600b0013 ffffffff
[<c07e2980>] (__irq_svc+0x40/0x70) from [<c00d553c>] (attach_task_by_pid+0x0/0x4b0)
[<c00d553c>] (attach_task_by_pid+0x0/0x4b0) from [<7c720cfe>] (0x7c720cfe)
CPU0: stopping
[<c0014e0c>] (unwind_backtrace+0x0/0x11c) from [<c0013748>] (handle_IPI+0x100/0x1d4)
[<c0013748>] (handle_IPI+0x100/0x1d4) from [<c0008658>] (gic_handle_irq+0x100/0x10c)
[<c0008658>] (gic_handle_irq+0x100/0x10c) from [<c07e2980>] (__irq_svc+0x40/0x70)
Exception stack(0xd0a67ef8 to 0xd0a67f40)
7ee0: 00000000 00000000
7f00: 00000000 c0f40f80 00000202 00000000 fa003000 d0a67fb0 0000000a 00000005
7f20: 3f2f9f3c 41a4cb28 c0d42c80 d0a67f40 c008ce90 c008c83c 20000113 ffffffff
[<c07e2980>] (__irq_svc+0x40/0x70) from [<c008c83c>] (__do_softirq+0x64/0x27c)
[<c008c83c>] (__do_softirq+0x64/0x27c) from [<c008ce90>] (irq_exit+0x48/0xa0)
[<c008ce90>] (irq_exit+0x48/0xa0) from [<c000f184>] (handle_IRQ+0x80/0xc0)
[<c000f184>] (handle_IRQ+0x80/0xc0) from [<c00085e8>] (gic_handle_irq+0x90/0x10c)
[<c00085e8>] (gic_handle_irq+0x90/0x10c) from [<c07e2b3c>] (__irq_usr+0x3c/0x60)
Exception stack(0xd0a67fb0 to 0xd0a67ff8)
7fa0: 18000000 00400000 00000005 00000007
7fc0: 6ed3fbea 6d491810 41525438 000075b5 41544b80 00000005 6d4917fc 41a4cb28
7fe0: 000000b5 becf5428 00000000 415478cc 200b0010 ffffffff
Rebooting in 5 seconds..
Going down for restart now
pm8921_turn_on_19p2mhz_clk: Kick Start
check battery fet
pm_chg_batfet_get: get CHG_BATFET_ON=0