I never updated my Eris to 2.1 (heard too many horror stories), but now I'm having this issue all of a sudden, with the original 1.5 still running my phone... The past couple of weeks, my phone will randomly turn its self off, then back on. Does anyone else have this problem? I can't even pin point a common factor either. And my HTC Sense has been crashing too. I don't have more than 10 apps installed, so I'm not sure what can be causeing this problem. 
I do apologize to hear that your device is not functioning properly, maybe you do need to update your device from the 1.5 to the 2.1. The 1.5 may be the cause of the technical glitches in your phone. Try updating your device to see if that resolves your issue. Hope this helps!
