I know it's not a bull, but one of my friends has a windstar with the 3.8 v6. He recently got the entire exhaust system replaced (including one leaky manifold) and then blew a rad hose. He had the hoses replaced with the thermostat and the sensor for the gauge. Now the gauge constantly goes up to the very high end of the normal range and then falls to the middle then goes back up and back down repeatedly while driving. I am suspecting the new thermostat temp is either too high or low for the engine but the mechanic says no. I'm just concerned he might warp the heads or blow the headgaskets. The van has 234k+ miles on it so it isn't exactly practical to fix any major problems and he really can't afford a new car right now. Any suggestions about what it could be?