There isn't much wrong with 80DegC constant while moving.
I can't remember off the top of my head if that model had a heater booster pump or just a valve for the heater.

If you have a booster than you'll need to check that it is working.

Does you heater work ok when the engine is at 85 DegC?

__________________
Alex.

MB Tech
Sydney, Australia
Volvo 122S
W201 190D 2.5 manual
W202 C240
W203 C32