I noticed that the iPhone 4 (placed in the Tomtom car kit) is getting very hot during video recording inside the LapTimer. Yesterday I tried to record a longer video, after aprox. 20 minutes the phone shut down during recording! As I took it out of the Tomtom kit, is was that hot that I burned my fingers

This does not happen while recording videos with the built in video app. Any ideas what the problem could be?
It's a iPhone 4 with 16Gig, no other app running. LapTimer is the acctual pro version.
Oliver