Are you using MultiUserHack by any chance? There's a conflict with that which could be causing this problem - normally it brings up an error message, but the Treo's don't display error messages on resets (since they're designed primarily to be phones, and hence they want to get the thing reset and reconnected as quickly as possible).
If deleting MultiUserHack doesn't fix this, which cellular carrier to you use? If it's Cingular or T-Mobile, after the reset go into the Phone application and dial #*377; if it's Sprint, dial ##377, or if it's Verizon, dial #*#377. If you're outside the US, use ##377 if your phone is CDMA or #*377 if it's GSM. That will tell you exactly what the error was that caused the reset, and if you post that here I can give you some more specific advice about what to try next.