First of all, I know nothing about the inner workings of a guitar amp. My Marshall MG50 has decided to crap out on me. When I turn it on initially, it works just fine, then it starts to phase out for a few seconds, then phase back in. What i mean by "phase" is that it doesn't just shut off instantaneously; rather, it acts as if I am turning my volume knob up and down. The longer I have it on, the higher the rate of phasing. I don't feel like spending money to get it repaired, so I want to open it up and try to fix it myself. If anybody can tell me what I need to do, that would be great.

p.s. I'm sure it's the amp that is causing problems, not the guitar.