What do you think putting the speakers on the 25 volt output is going to prove? It's just another tap on the output transformer. And your voltage/current measurements mean nothing, it ain't that easy to measure wattage else why would you need an impedance meter?
If you are absolutely sure that this is not coming from an input or dirty controls are the culprit, if you still think this intermittent problem occurs when the amp heats up it's the amp. And no, overloading the amp won't cause this kind of problem. Repair or replace.
-Hal
