Thermostat problem??

May not be your thermostat at all. Until your engine heats up to operating temp, the thermostat keeps the coolant recirculating through your engine. Finally, when up to temp, the thermostat opens and allows coolant to flow through the radiator. This may be what is causing you to feel a drop in temp coming from your heater. You should make sure your cooling system has the correct amount of coolant and the coolant is the proper 50/50 mix of coolant and water. If that is OK, then look at whether your heater controls are working as they should before going to other items like the thermostat. Your problem does not sound like a water pump issue.

Bleed the cooling system.Im pretty sure you have two bleeder screws.Check you tube to see how to do this.Its simple.Just be sure and put penetrating oil or WD 40 or similar on the bleeder screws awhile before you open them.They can break off. I dont want to scare you away from doing it just forwarn you. Pockets of air inside the system will create false temp readings as they pass the coolant temp sensor. A pocket of air in the heater core will create a no-heat situation. Your motor is famous for this.Did someone recently replace a cooling system part?Or did you find the system WAAY low on coolant and ad alot?These are circunstances that lead to air pockets

Im new to navigating this site.I just scrolled down and saw you previously mentioned you changed cooling parts and bled it.BLEED IT AGAIN! Park with the nose up hill,jack it up as high as you can. Bleeders open.Id even begin with the cap off. Let the motor idle but rev it up also. Youd be amazed how tough it is to get all the air out.Taurus's are like this also.

What would make a hose collapse when the car is off and expand badly when started once it has been running for a while all works fine