Hi all. First time user and posted a thread last night. Received some great advice regarding the forum.

I have a 1997 Ford F150 4.6l and having problems with heating system (I think). When you let the truck idle and temp gauge goes to normal, then you drive it and the gauge drops to cold. After awhile it goes back to normal. The resevoir fills up and wants to overflow, I losen the cap, it drains and returns to normal.
I've replace the water pump and new thermostat. I went and had a rad system flush. And still the same problem.

Any ideas what this could be?

 

Could be a blown head gasket. Have the cooling system tested to see if the are combustion gases in it. Good luck!