Overheating madness

I have a 2000 Venture...been having some overheating issues over the last six months. I don't drive the van, my wife does so most of this has happened with her driving. The first time it happened, sitting at lights would red line or go that direction, then when she would start driving go down. Took it to the mechanic and it was a fan relay. That fixed it for a while...then it started to do it again last month. Took it back...he replaced the temperature sensor. I have had it back for almost a month and my wife just called me and it did it again. It red lined on her and the coolant light came on. She turned the van off....when the light turned green she started up and got the van thru the intersection into her work parking lot. The temperature already was back to half or just a little over. If the van was truly overheating...it couldn't have dropped temperature that fast...could it? Could a bad thermostat or something cause a quick rise and fall in temperature like that? The mechanic did change the oil the last time as I feared the worse with froth in the oil...but he said oil was clean when he changed it.....this is driving me nuts.:eek7:

Wife just got back in and she said the temperature guage went up and down both while driving and sitting at the light. She said it went up faster when sitting at a light. From what she described the needle would go to somewhere between halfway and the beginning of the red zone. I just went out to the van, the overfill tank is at the line right where it should be and the oil on the dipstick is clean as a whistle. I would think if the head gasket was going and coolant going into the overflow tank I would have lost some somewhere along the line......

it s the thermostat for sure,i had the same problem

All of my experience with these engines points to a bad head gasket when it acts like this. I've gone through 3 of them (that's 3 vans) and all three did exactly what yours does. Each took about 6 months to finally get to the point where it was obvious. I didn't want to hear it was a head gasket, but that was the problem.

Its cheap to change the thermostat, not fun, but cheap and I'd go that route too, but I think in the end your heads will need resealed.