Seems to be sensor related. If it's not sensor related it's seems wierd that it runs good till it warms up. I'd say it's got something to do with the temp sensor that goes to the computer, or possibly the thermal valve is bad.

I'd check the thermal sensor first. There are two thermal sensors, one for your dash gauge and one for the ECU.

By the way I am assuming this is a 2.6, 4 popper?

A gun in the hand is better than a cop on the phone.

98 Passport 33's, Supercharged, Calmini Bumper, rockbars, diff drops, Teralows, 4.77's, Aussie and ARB lockers, Safari snorkel, Optima red top.