honestly buy a new cd player trying to fix these things aint worth it
could be wiring
could be loose parts from wear and tear
or loose connections but if you want to spend the time searching for the fault good luck as they dont have arrows pointing to the faulty bit and just by looking 9 times out of 10 you'll miss the problem.
sorry to sound like a prick but have tried to find faults before and ended up just getting a new deck and that always solved the problem.
three thing in life are guaranteed
taxes
death
and car repairs