RogerAF

I suggest checking connections around the power tubes and the output stage. If the effects send continues to work, then your preamp stage is likely OK. Look for cold solder joints. Run a signal through the amp and wiggle wires etc. Also check the tube sockets, sometimes they have internal fractures that aren't apparent. It could be something that only shows up once a certain temperature is reached. The way you describe it as working and then going down after a period of time makes it sound like it could be happening once the amp warms up. You could try spraying Freeze-It on various components and seeing what happens.
Good Luck & Good Hunting.