Hi Curt, I've also had some bad experiences with Mackie mixers. I've got a friend who has a SR24-4 who does a lot of live sound gigs. He's fried some stuff due to ultrasonic oscillations that pass right through the Mackie. Based on your description, before blaiming the mixer, I would suspect power problems or even some unauthorized person messing with the system. I think I'm missing something.
Electricity does always follow the rules.
A few years back, a church was hit by lightning. The current followed up the ground of a new church that was only opened 6 weeks. Toaster, Computer, Alarm System, Sound System and everything that had a 3 pin AC plug was destroyed. The only items that were not damages were AC products with only 2 pin plugs. The NRC of Canada was called in by the insurance company.