Restore Mode

I couldn't find this specifically anywhere so I figured I would give it a shot. I just sold my old 3g, it was working great, good battery life, and running quick. The kid that bought it emailed me a couple days later and told me that give or take every 4 hours it shuts down and then goes into restore mode. The only time I have come across this is when the phone has been poorly jailbroken (which he says it hasn't). Since I'm not sure what could cause this I'll just include everything. When we sold it it had the most current iPhone software loaded 4.2.?, we had it synced with a mac and he is using a pc, we pulled the sim card and he replaced it with a sim card he already had. I have asked him on multiple occasions what software version he is running, what version of itunes, age of sim card, etc. just to get all the info but he hasn't answered any of those just said it's not working. Any ideas on what could cause this?

Quite simply, you can't offer any help without the answers to the questions you've already asked. If he won't answer, there's not much you can do, except ignore his responses the way he does yours.

I'd tell him it worked fine when I had it, if he wants any troubleshooting assistance he has to answer your questions.

I just heard back and he is running the most current version of itunes (pc) and the most current software on the iphone. He is using a non-3g sim card if that makes a difference. Anybody have any ideas with this information that might cause the above mentioned problem? If not any questions I can ask that might help?