My phone does the same thing. I've had it for almost a year and lately it has taken to restarting extremely often. It's never happened during a phone call though. It also likes to turn itself off, and then not turn back on. It won't recognize a charger and I can't turn it on. This has happened to me three times since September. Last time it happened, I took it to a service center to get it fixed. They asked me all the questions about whether it was water damaged, etc., but it's never even been touched by water. They took it apart and cleaned it up (they said it was really dirty) and then looked at me like an idiot and said it was working just fine, but low on battery life. (It also has no modifications to it, aside from a few ringtones and one game downloaded from the nextel website)
Has anyone else had a similar problem? This is my primary (read: only) phone and it's hard to go without it and I'm not looking forward to shelling out several hundred bucks and having to extend my contract to get a new phone.
Sorry this was so long, but I'll take any help I can get.