1989 Dodge Shadow overheating

Okay, here goes. Recently, my car began to have a problem with the cooling system. The temperature guage would rise toward the high mark when at idle, then recede to slightly less than half (normal cooling) when I would begin to move again. If I put the car into neutral and revved the engine, the temp guage would again go back to normal. Also, the temperature of the air coming from the heater would rise and fall the same as the temp guage. My mechanic said this was most likely a thermostat problem and changed the thermostat. It continued to happen, so a couple of days after that, he replaced the drive belts and the water pump. Now, the car continues to do the same except for one thing - when the car is started after totally cooling down, the temp guage will slowing rise (as I drive) until it is at the complete overheat temp. I looked and the cooling fan is operating and the overflow tank is full and overflowing. It will stay this way for a couple of minutes then suddenly the temp guage will go down to about the one-quarter mark. As I drive after that it will go back to the behavior it was having of moving up and then back as driving or idling. Also, the car has a new radiator. Any ideas as to what is happening and why? And what I can do about it? This is getting very frustrating and I want to avoid spending more unnecessary cash.

Thanks,
Dave H.