Live near salt water? Could be corrosion on any number of connections. Here in the Chesapeake Bat area we have this problem from time to time.
If you have a lot of external stuff connected to the A1200 take it all off, and see what happens - add things back one piece at a time.
Somewhere in that process you may be able to get SnoopDOS to tell you something...
double check your PSU too.
