My 99 Jeep Cherokee has recently started having heat issues. The heater works, lots of hot air when the car is moving. But if the car is idle there is no heat. If im driving up to a light the heat will be fine, but as soon as i stop at the like the heat begins to cool, by the time the light changes its just cold air. The Jeep also seems to over heat when sitting still, the temp will get to about 225F and then the fan turns on and the temp goes back to 210 and the fan turns off, until it heats up again. If im sitting still and give it some gas, the heater will come back. So its definitely RPM based. Ive been doing some reading online and was reading that the heater control valve is vacuum controlled and so is the thermostat. Before I start replacing parts i want to make sure Im going in the right direction, so any advice?