Parts came in last week and I recapped the power supply board. Presto! No falling into protection now for four days, so I believe that has solved the issue. I tested all of the old caps after taking them out. Most were testing high between 15 and 20% of original capacitance. The large one on the power supply board, C16, tested 27% high, and C12 and 13 were high by 29 and 33%. So, not sure which one specifically, but one or more were causing issues.
I also went back and tested all the caps I took out of the power amp and protection boards. Here again, all caps were high by 10-20%. A few were over 30% high, and one was 63% high.
Just out of curiosity, what is considered an acceptable range of capacitance to be considered "in spec"?
I ordered replacements for all of the electrolytic caps that remain (save one that was on back order), so I can finish the recap when I have time. I agree this is totally worth it. The 8100 is such a wonderful sounding amp and versatile with all the separate controls. Can't speak more highly of it as a Pioneer product.