I am also thinking this might be wifi problem. I had to exchange my phone over the weekend because of camera problems.
While setting up my phone I had wifi on and some apps wanted to update, went ahead and started updates and shortly after the phone rebooted. My phone also rebooted while downloading more apps.
Tonite while talking on the phone it rebooted again wifi was on, after it rebooted I noticed it was updating an app.
Before I could even send the error message to htc the phone rebooted again while the app was been downloaded.
Really dont know what to do at this time.
Sent from my Evo