2001 Outback - Tried “everything” help!

Okay. So I’ve got a 2001 Outback that is
giving my mechanic and I a run for our money.

It overheats but ONLY when the heat is turned
on in the car, after driving highway speed,
then stopping. More granular details below.

We have replaced:

- Both head gaskets
- Thermostat (twice!)
- water pump & timing belt
- radiator

Here’s what happens.

Driving in the city, heat on - no temp change
Driving on the highway, heat on - overheats
when idling, not while driving
If I turn the temp to cold, the gauge goes back
to normal, instantly.

What the heck could this possibly be? And
yes, we have already vacuum sealed the
system so there’s no bubbles.