Amp sounds great for about 30-60 seconds, then just fades away to a low quiet volume. It appears something is heating up, then opening up/failing. I have checked all the solder joints and they look good. I have also ran the amp with chassis open, and pushed around on the various components to see if it will come to life. Nothing. Banging it on the top, etc. does nothing also. Anymore suggestions of where to look?
Have you confirmed the power supplies are intact? Tracing the signal could help if we knew if it was the preamp section. Have you tried to tplug in the CD input to see if the amp cuts out there too? Make sure to test that way too so we know if it is on the preamp or power amp side.
http://www.amparchives.com/Amp%20Archives/Marshall/Schematics%20&%20Layouts/MG%20Schematics/Marshall%20MG15CD.pdf (http://www.amparchives.com/Amp%20Archives/Marshall/Schematics%20&%20Layouts/MG%20Schematics/Marshall%20MG15CD.pdf)
The CD input doubles as a preamp Line out, so as well as injecting a signal to see if the main amp is fading you can extract a signal to another amp to see if it's the preamp is fading, that way we can be sure which section is faulty, but checking the power supplies are holding up is an important first step.
Tried the headphones, guitar still sounds the same, low volume, distorted. With guitar plugged into the CD jack, it is working. Not real loud, not real strong, but clear and pretty good. So I take it the problem is in the preamp section and not the power amp section.
Yup certainly sounds like a preamp problem. Have you confirmed your -/+15v rails? Check IC1 and IC2 pins 4 & 8 for the 15v power supply. If that checks out good then it is on to signal tracing to detect where the signal might be cutting out.
Edit: Also we want to look at pins 1 and 7 on those IC's (TL072 chips) to make sure what voltage is measured there too. If you have high millivolt readings on pins 1 & 7 of those chips then we might have bad op amps. Are any of the chips getting hot?