Help: Car drops temperature and runs cold when driving on highway at 55+MPH only.

[cg312]

Help.

I've recently had my alternator replaced (new) and began noticing that my cars temperature drops significatly when driving on the highway at 55+ MPH. I notice this only happening when driving on the highway and when driving on the street (20-30 MPH) the engines temparature runs normal...btw, it's Winter here in Illinois.
I once stopped the car when the temperature droped and checked the coolant hose - didnt feel hot at all compared to when the car runs on the streets.
Since noticing this problem I've had the following replaced; new radiator, new thermostat, new inginition cable.

Car startup warmup is fine, take about 3-5 minutes to fully warm up.
Again car runs hot and heater works fine only when driving on streets.

Car year: 99
Mileage: 111000

Thanks.

[comp]

sounds like a bad thermo. to me??? can you pull it to test, try some card board in front of rad. to block different amounts of air to test

[comp]

P.S. forgot!!! make sure you don't have a air pocket in the system

[algoesfast]

I agree with comp it sounds like your thermostate is stuck open.

[cg312]

Thanks for the replies.

You guys were all right stuck thermostat and air pockets.

Well after days of frustration I decided to take it to a Chevy service dealer to take a look at it knowing they were going to charge me an arm and a leg ($180 btw) for the service. Well turns out it was a falty thermo.....
Guess I should have gone there in the first place than the hole in the wall mechanic shop down the block, which btw assured me the thermo was good.

Anyhow, thanks all for the replies.

[comp]

180 bucks i got to get my
garage opened i can do it for 175

[heatherannie22]

Thermo's can be bad right off the bat, I've heard boiling it before putting it in is a good way to test them. I've seen them bad when they're brand new, right out of the box.