ARC repair needed

What does, "is going out" mean?

They’re only worth around $700 or so and can be unreliable. Maybe the Mk2 is more reliable although your experience suggests not.

The right channel starts to fade after about 10-15 minutes.  It grows faint by about 30 minutes.  

Try Pyramid Audio (Texas) or United Radio (Rochester NY)

Problem is the transistors in that amp are obsolete and it's very difficult to find an equivalent that will work. Transistor amps are very intolerant to transistor substitutions no matter how close the specs are.

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.