Does the car blow any smoke when its doing that?
I know that those cars (at least some) had real bad problems with the head gaskets. Sounds like there is some coolant collecting in a cylinder and you have to keep it running until it burns off.
Can't remember if you've said you did the gaskets recently.
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1997 Mustang GT "The Freak" - 13.80 @ 101.70, 2.07 60'
1995 Honda VFR750 - not much @ really fast (actual data pending.)
1964.5 Mustang 289
Rice Haters Club Member #13