Overheating

The temp goes up when we stop or are going really slow. Almost as soon as we take off again, it goes back down. The heat works only when we're going at a good, constant speed for a while. When you stop/slow down, the air is cool again. Didn't know if any of this makes a difference, just wanted to add a little more detail. Thanks

We checked the water pump, it is good. We changed the thermostat. Now it gets hot while driving, instead of while idle. If you turn the car off and open the bleeder valves on the coolant system and let air out, the temp goes down. I don't see how air keeps getting in though. The radiator cap doesn't really look so great, so we are thinking of changing that. I have no idea!

I have 2002 Montana and am having same problem. Temp gauge fluctuates up to very hot when stopped and then goes down normal after driving. And just in the last couple days, noticed no warm air when heater is turned on. Can anyone give me direction what to do to fix? Thanks.