I have a Pioneer SA-1270 four channel amp driving two JVC SK-303 speakers (80's anyone?

) that I'm having some connection issues with. Sometimes the amp will work perfectly with no issues, and other times...not so much
When it doesn't work right, it cuts out on one or both speakers, will completely stop driving one or both speakers, sometimes has some crackling, and decreased volume obviously. I can usually affect how good or bad it is by moving around the button that turns on and off the A channel (the one I'm using), but if that doesn't work a good slap on top of the receiver will do the trick. The B channel also does this just as bad, if not worse than the A channel. It does this on all inputs as well.
Is there any way I can open this bad boy up and see what's going on? Throw some ideas at me! I'm not opposed to messing around inside there. I know I could buy a different reciever, but I like this one and it's just the garage stereo so I'd rather not spend a lot of money on this setup. Thanks guys!
