AMps cut out for a million reasons.
Isolate the problem. Send the effects send signal to another amp. Does that signal go away when the one coming out the speaker does? If so, the problem is in the preamp. If the remote amp keeps going while your speaker drops, then the power amp is the problem.
Plug a spare cord from effects send right back to efects return and see if that makes any difference.
Inspect the solder under all the jacks and the impedance selector switch. Switch the impedance selector back and forth a few times to clear the contacts - any help?
Check the rectangular cement resistors - the ones up on stilts - any of them coming loose? ANy rocking back and forth too easy?
Are you using a real speaker cord and not just a spare guitar cord for your cab?
Have you tried a different cab and different speaker cord?