Tom,

Too many variables.

Run the test you just did - heating engine up to 100-120, then turning heater on.

Do not use acceleration (higher fan speed) to drop temp, use heater only.

If temp drops when you turn on heater, you need to flush or replace radiator.

Do run a search here if you decide to flush, as, although I most often try most economical solution first, I seem to recall a lack of full success in flushing by some folks ... at least I picked up enough negative impressions that I decided to simply replace radiator - which is not that difficult on these cars.