I'd also go to 50:1 ashless, though that's a matter of taste. Your temperatures seem okay, but your symptoms point to an overheating issue or ignition timing/performance problem. IF CH replaced parts on your ignition, make SURE you get at least a 1/2" air-gap spark when flipping past TDC with the plugs out. Both sides should be equal.

This is the second post, and third engine (including my own) that seems to be having ignition problems with the CH units. Sagging cn be caused by a weak spark, synchro-spark module not fully advancing the timing or getting hot and degrading its performance, or one cylinder dropping off line.

Keep us posted on what you find.