Amp gets quieter and quieter as I play....

If it gets to the point of inaudibility, I can shut off the amp and immediately turn it back on and it will go back to normal volume...

That's definitely not normal. Take it to the place you bought it, bring your receipt and warranty if you have it, and tell them what's going on. They should fix it or replace it. How long have you had it for? If it's relatively new, tell them you want a new one because it shouldn't be messed up so fast. Make sure your power chords are plugged in tight at the amp and at the outlet just in case... if that doesn't help, bring it in.
-Justin

I bought it on eBay.  I e-mailed the person I got it from to see WTF is up.  Would a store let me bring it in.  I doubt it...

Anybody else have any suggestions?

BTW, it will play a little longer with my ZOOM 505II plugged into it.

I called my dad, who is a Master Electrician, and described your problem. I forget the term that he used, but, he said that it sounds like a short that occurs as the amp warms up.
Like, there is a transistor or other component on the curcuit board that is broken at the solder-joint, and maintains contact until it gets hot and expands.

If that's the case, it's a simple repair. Just a matter of finding the bad connection. Any electrician should be able to do this for you. Good luck.

Looks like some arsewipe decided that it was cool to sell broken equipment. Try to get it repaired, and send the bill to that dudes house.  >:( >:( >:(

My guess is you're stuck with the bill unless it was new and the manufacturer will cover it.  >:(

Here is a copy of the e-mail he sent me in response:
it would shut itself off??? Dont know what is up with that. It was working fine when i had it hooked up in my car. Did you check and make sure everything was hooked up right and everything was tight? double check the fuse also to make sure its in all the way and that your ground wire or power wire doesnt have a problem. Let me know if you find out what is causing it. I even tested it not long before i sent it to you and it played fine so im not sure what is going on with it. Definately let me know if it persists though.

He had it hooked up to his car.  WTF?!?!  Anybody think that could explain it?

It could be a few different things, I believe Preston was referring was called a cool solder joint, thats when after a little while the joint expands and either makes not contact or very little, A couple other things maybe the output transformer for the speaker, the power supply transformer (step down transformer), which steps the 115 down to say 5 volts. Or a capacitor maybe discharging.

Without hearing it, testing it and checking voltages this would be a "WAG"  
(Wild A*s Guess)

That mail quote sounds like he may have sold a car stereo amp and is confussed. I would continue to conact him, If he gives you the run around contact e-bay. they generally take action. It's called Internet fraud.

joe

Would say forrock_stars on the right lines here, definately a capacitor problem - a cool solder joint would not do as described if you can switch it off and back on immediately. As forrock said the most likely one of the capacitors on the power supply or on the amplifier output stage is discharging. Apart from a faulty component, dampness or dust can also cause this - it would be worth checking this out by opening the amp box up and cleaning dust off with a paint brush or vacuum cleaner and checking for moisture (REMEMBER TO UNPLUG IT FIRST).
If this doesn't work or your unsure, get it checked out professionally - it shouldn't cost too much, ask them to give you a quote for repair first - send that on to the ebayer and ask him to pay for it. If he refuses give him bad press and inform ebay.