Heat only works at 3000 rmp after car has come up to temp. If rpm drops below 3000 heat temp at vents will drop. Have changed the thermastat with no positive results. Temp gage has shown a quicker rise in engine temp. This however seams to have no relevance in the in cab temp.

If it is a manual system--sounds like coolant low ie: air bubble in system. If it is ATC system, Check Codes, if no failure codes, coolant may be low--air bubble in system--most Durango's have air bleeders in system.

The heating coil is probably getting plugged up. You may have heat at idle or slow speeds, but once you get going, there is not enough coolant going through it to stay warm. Get the system flushed or replace the heating coil.

could be an air lock in the heater rad, back flush with a hose to clear.

try the heater core it might be pluged up or the pump isnt circlating

Most likely low coolant if the blower is still working just not putting out heat

Sounds like the thermastat is getting stuck, try changing it out with a new one.

radiator low in water.

I think you're right. If you have a water temp gauge and it reads normal temp but you still have no heat at idle then you have another problem. If you just have a temp light then I'd replace the thermostat, it's easily done. Just go to your auto parts store and buy the Hayes manual for your car.

check the thermastat

Are you turning on the compressor (as in turning on the air conditioning) or just the vent? If you're turning on the compressor and the revs drop, that's normal. The compressor is driven by an engine belt, and when it is turned on, it puts additional drag and load on the engine, causing the revs to drop.