It could be either a problem with the 'peg or the amp. It could also be a wiring problem, including the classic loose wires in the docking connector problem.

First, check to see if you have the same problem with the 'peg plugged in indoors, using RCA cables to the home outputs. If there's no problem that way, it's unlikely to be internal to the 'peg, and it's time to start looking for wiring problems or amp problems.