I had the same problem after the 2.2 update, contacted HTC and they told me to do a hard reset but before that I did my own investigation and discovered that when it was set to auto-sync, I'd get the issue and if I turned auto-sync off and did it manually, I don't have a problem.
I've been auto-reboot free ever since!
Apparently though, someone else told me their reboot issue wasn't due to auto-sync, so maybe it's just me but honestly, it's been fine since I turned that one thing off. No other issues with 2.2 at all.
Perhaps you are best to stay with 2.1 for the moment if you're happy with it? I can't guarantee that my fix will also be your fix!
alis volat propriis