i have the same problem with my iphone 2g. went from 1.1.4 to 2.2.1 and noticed it keeps rebooting on me. at first I thought maybe my phone is overheating, so i took it out of the case, but it still reboots on it's own. It looks like a memory leak problem...
i've already unlocked it twice using quickpwn on windows xp. First time i only added cydia, and i did a sync with contacts, photos, and old data...that kindda messed things up, so I unlocked again. (wasn't paying attention if it was crashing alot on this first run).
Second time i added both Installer and Cydia. Then when I ran cydia for the first time it asked if I want to update all files, or just essential...I clicked ALL and it screwed up cydia. So i plugged in quickpwn to only re-install cydia and cydia worked again. But noticed that it crashes a whole lot. This second time, i didn't sync my contacts, sms, photos or anything.
I also synced with Itunes for free apps and it works. Some apps will give a "verify" problem, but I would just delete it and download agian.
Problems that I constantly have with my iphone on 2.2.1:
- constant crashes when opening sms, mail, apps. web browser seems fine.
- once i open one app or mail and it works, then turn off the iphone...wait awhile, and sometimes it just reboots on it's own. same with having one app open for awhile and pressing around, it would just reboot.
I'm going to unlock again with quickpwn, but this time im only gonna add cydia (i dont use installer, so screw it). Will post another reply in a few days with results.
sorry for the long reply...