i can only guess at this point i need to replace the temp sending unit, but here is what is happening-

concerning 1979F250-351M-C6; only been driving this truck 5 months; starts good, runs great; noticed the temp gauge holding the needle below the 'COLD' side when it should be (actually is warm) indicated 'NORMAL' temp range, while running down the road (55-60mph) and when i let off the gas pedal the needle begins to rise and if i am at a stoplight or otherwise sitting still with the engine running the needle will normalize to a 'median' position in the 'NORMAL' temp range. as soon as i move again and get up to speed, the needle will drop below the 'COLD' side again. this continues in my 'stop & go' driving to and from work. the engine coolant is fine, t-stat si fine, and i hate to replace the sending unit unless i know it is the problem. any ideas?

 

I'm thinking water pump and sensor. The hot\cold cycle when driving and then idle sounds like the pump is not moving the coolant well at idle. The low end spectrum of temp could be sensor. If you could I would test the actual temp to see where it is at. Also with that quick of a swing it still sounds like the t-stat is not doing it's job.

 

Have you noticed the oil pressure and fuel guages rising and falling at the same time? If so, it could be the voltage regulator on back of the cluster is bad.