There is a possibility you got a bad fan clutch, as the Hayden one I got was never very good, never moved much air and bit the big one after less than 2 years. The fact that your temperature goes down when you start moving is a sign that your radiator is probably working correctly, and there could be 2 problems. One, you got a bad fan clutch and your electric fan can't keep up on it's own, however I'm not sure about that. There was a time when I took off my fan clutch and bolted on the pulley and drove around with just the electric fan and it took an AWFULLY long time to heat up.
The other possibility though, is your water pump. If the impellers get eroded away, they are just sort of little nubs that don't move much water, especially when idling. Revving the engine can get them moving enough to bring the temperature down. If you have the original pump, you may be amazed at what the impellers look like (or don't look like).
Obviously there could be something else, but those 2 options seem the most viable.