I have a 3GS that someone gave me because it wouldn't charge or be recognized by their Mac, so they got a new one and had this one laying around. I put it into recovery mode and did a factory reset and it's been working flawlessly for almost 4 months.

They were also one that had a ton of apps loaded. I don't know if that was the cause, if it was a buggy app, or if it just needed a full reboot. Is yours jail broken?