Mine does that too!
Very annoying. It seemed to correlate roughly to when I installed a special version of EnergySaver (it was 1.2a, because 1.4 wasn't working for me; basically it was 1.2 with a bugfix in it).

At around the same time, I noticed my battery life getting shorter. So I switched to my newer battery. I still got the shutoffs a few times, but it seems to be much less now.

Also, I've been trying to witness one of the random shutoffs, but they're so random that it's quite difficult. I got within a few seconds once, and I'm pretty sure it's not using the normal power-off routine, because I didn't actually see the LCD say "Powering Off". Does anyone know what happens when the 7135 shuts off due to low battery level? Does it show "Powering Off"? Maybe a bad battery (or contacts?) was causing the sporadic shutoffs.

I don't use SMS, and my charger seems fine whatever with method I charge. I usually plug the charger directly into the phone, but occasionally plug it into the USB cable. I don't use the cradle much.