Hey caponi14,
It seems like you need to clean your amp, to put it another way, you need to clean the pots on the amp. Make notes of your settings, and remove the knobs. Use compressed air to clean them, but don't get the cheap stuff, get something with a name. You also want to make sure you can use it for audio, and that it won't harm any other parts. After spraying them, give them a few good twists, spray them again, and give them a few more twists. This should remove the dust, or whatever is causing the problem.

This would be the first thing I would try anyway, and you still want to check all of your cables too.

Let me know if this helps, and I hope it fixes it.