1998 Pontiac Grand Prix My car barely blows out heat and th

Heater problem
1998 Pontiac Grand Prix 6 cyl Four Wheel Drive Automatic

So almost 2 years ago my head gaskets were replaced, and now almost 2 years later, my car won't heat up and if it does it takes a LONG time, and depending on how fast I'm going the termostat may rise or go down, the lower the thermastat goes, the warmer my car gets. If I'm driving in town, my thermostat will rise, won't quite overheat but then when I drive faster or reve up the engine it will go down. If I drive 50+ mph the thermostat will stay low and the car has no issues warming up. I've had my thermostat replaced, which was a good thing because it was bad anyways, but the problem is still there. I just remember 2 years ago kind of the same situation and I put almost $500.00 and come to find out it was my head gaskets which were over another $1,000.00 to replace. But with those symtoms, whenever I went faster the higher the thermostat and it was blowing out white smoke. So I honestly don't think that's the issued, because the car acctually doesn't overheat like before, and when my head gaskets were blown, when I started my car the thermostat blew straight up as now it doesn't. I just don't want to end up spending thousands of dollars again. Please help if you have any advice. PLEASE! It's winter and I live in ND and need heat.