You need to describe what it does when it fades out. You appear to have isolated the problem to the receiver, but you can go a little further. Plug a CD player into an auxiliary input on the receiver and see if the problem persists. If the problem persists, the problem is in the high level section of receiver. If the problem is not present with the CD player, the problem is in the phono stage. Is the fading channel left or right? Does it just die out slowly, or does it jump in volume? Does it make noises before it fades? Does it distort while it fades? Does it distort in the low-volume mode? Does it reduce in volume or go completely dead? Answering these questions will help identify whether it is a power supply problem or a problem in the signal path. You can have a bad device, a bad power supply, or a device that is erratic.
You have one good channel. You can compare voltages and signal passage between the good channel and the fading channel, and this will identify the problem stage. Most old receivers use discrete devices rather than IC chips, making them relatively easy to service. Open it up, wait till it fades, and then spray some circuit cooler on the likely devices and see if it comes back. The device that brings it back when you spray it with circuit cooler is the bad one.
Not much help here, but a further description of problem might assist. Service Manual available here: http://cgi.ebay.com/Original-Service-Ma ... 3ca6f18c9e
Don't know this vendor, but might be a help to have manual if you want to fix it. Schematic provides aid in diagnosis.
Good luck from the old inquisitor,