ARC repair needed

In Southern California, you might ask George Meyer AV. They do repairs but I don't know if they'd have the parts and information needed for your specific model.

Sounds like a thermal issue on one side which is invoking a protection circuit. Can you swap the speaker leads to see if the fault moves to the other side? If it does, it's a speaker or cabling issue. 

I tried swapping both input leads and outputs to speakers.  I tried reverse on the preamp and inverting in the amp.  It’s always the same output.  I suspect it’s either a solder or a capacitor.  

You can use freezer spray to check for cold joints.

High End Audio Repair in Phoenix is great.  James is the owner.