Stereo cutting out at low listening levels. HELP!

Here is my set-up first of all...
* (1) 5-channel amp
* (4) 6.5" speakers runing off channels 1-4
* (1) 12" polk sub running off the 5th channel
* Sony CDX-MP80 CD/MP3 player

I had posted before saying that my stereo was cutting out and making crackling noises, etc. When a CD was playing, it almost sounded like a radio station that wasn't coming in very well at all.

Anyways, I just noticed something tonight that I hadn't before. When I turn the stereo up to a louder listening level, the stereo starts to sound normal again. I did notice that when I would turn the bass knob up on the amp and it starting hitting pretty hard, the stereo would start cutting out again. What does all of this mean? It's almost like the stereo has to call for more power or something before it sounds good. I really need help with this as it's been doing it for a few weeks now and I want to be able to listen to my stereo. If you have any suggestions, I would appreciate it. Thanks in advance. (By the way, all of this is being done while the boat is off)