I've got a strange intermittent fault with an iPhone 4 16gb. Sometimes, but not always, taking a picture with the rear camera...
This mini circuit board for the thermal sensor is very delicate and stuck on quite firmly, so be very careful removing it. I ended up cracking mine in half. On re-assembly the system would run but the fans were on 100% until this sensor was replaced with a new part which was not easy to find.