ron mentioned the coolant sensor. i ran into a deal one time where the coolant sensor had film around the end bulb [lime deposits and anti freeze film] where it was in contact with the coolant. this film tricked the coolant sensor into a false reading, thereby causing havoc with the fuel curve and severe driveability issues. swapped out the coolant sensor and all was well. remember that some applications have a separate coolant temperature sensor for the gage reading in addition to the coolant temperature sensor for the computer function. some are combo sensors. i don't know what you have, so if changing the sensor, make sure it is the one for the computer function.