iPhone 3GS Keeps Rebooting
I had this issue with my Girlfriends iPhone 3G, as you say the only way to resolve is to a restore in iTunes. 

So are you and your wife running iOS5 or is the 3GS on another version?

Once that is done it is worth upgrading (if you haven't already) and make the iPhone sync with iCloud and store your backup there. This way if it happens again you can easily restore to an up to date version.

Hope that helps!