Have you checked the monovalve? I don't know what to check. If you haven't replaced it in a few years maybe just replace it.

By the time the coolant temp gauge is at 80*C there should be more heat than you can stand. Check if the AC compressor is engaged when it shouldn't be?

Do coupes have two aux pumps? Why would you need two aux pumps to feed one heater core? Is this a Euro model?

Sixto
87 300SDL