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?