[XPERIA X10 General] loneLEE:
Hey, had to create an account just to answer your queries...

If you mean by the phone not able to 'wake up' from its sleep mode, I experienced the same thing for the past one month...

For my case, I started one saturday with my phone acting this way when I did a reboot after a full charge.

Same as your phone, mine suddenly couldn't wake up from its sleep. I thought is was just an ordinary freeze, so I took out the battery to reboot it once more.

After that, the problem didn't do away. With an attempted phonecall which triggered the proximity sensor to deactivate the screen display, the screen couldn't come back after ending the call. So the suspicion of lags or freezings were ruled out.

The phone was basically working, just that the display wouldn't appear. This was supported by the fact that the LED lights between the hard keys were still functioning, and the faint backlight on the screen hows that the phone is indeed working.

But the unlock screen just wouldn't show up.

So, without a choice, I sent it for servicing.

First attempt, the phone had a 'software upgrade' (just reinstallation of the software). Didn't help.

2nd attempt, the technicians replaced the receiver hardware. Didn't work either.

3rd attempt, they did another software upgrade (I have absolutely no idea why they tried that again since it didn't work on the first time).

4th attempt, they replaced the whole motherboard, and I got it back last night.

And you know what? The problem didn't go away. But I didn't send it back for servicing, since replacing the motherboard IS already a very last possible solution, as my X10 is as good as a new phone.

So obviously it's the user's problem.

So I did abit of testing on my own.

With the microsd card taken out, the phone was perfectly ok. I spammed the power button to stress-test the screen. No problem. But when I re-inserted my 16gb card, the sickening problem was back.

So I suspected it might be the card that's faulty. Naturally, I tried it with my 8gb card which came bundled with the phone. No problem. Until I moved my files into it.

Narrowing it down, obviously it's the contents that is affecting the phone.

So I did a step-by-step trial:

I removed the video folder, cos I have quite a number of movies inside. So I thought it's the sheer sizes of the files which is lagging the phone out.

Wrong.

Next, I removed the music files.

BINGO.

Phone was working fine. And to make sure, I put the videos back. Screen responding nicely.

To enact my theory, I just dragged and dropped my 1600+ wmas into the music folder.

Voila! Case closed. 20 hours have gone, and that idiotic problem never resurfaced.

So... What actually happened???

Well, as I recall, this whole escapade happened after I used Media Go to transfer my music to my phone.

And Media Go acts like iTunes - it sorts the files into different folders categorized by the artiste names. So essentially, almost a thousand of folders were created in the microsd card.

I guess it's the immense number of folders that's making the phone try to read from the folders, thus lagging it and preventing it to work before the screen goes back to sleep.

For me, cutting the number of folders helped.

Hope my method works for you too =)

And pardon my long-windedness (+_+)