Seeking help with an amp problem

Hello. I bought a B&K EX-442 amp used on Audiogon. It was trouble-free for a few weeks. Now, when it's first turned on, the right channel doesn't play at all, but after an hour or two it gradually comes on, but after awhile it becomes silent again. (There is no sound whatever from the channel when it's not playing properly, not even static or hissing.)

I tested the preamp and sources, and there's no question that it's the amp. I've had many amps over the course of many years and I've never had an intermittent problem like this.

My thanks for any guidance anyone can provide.