Sooooooooooo....Hello ALL! Happy to meet you all. Hopefully there is someone around with some insight or ideas. Truck: 1989 Chevy Silverado 1500 Z71 Engine: 350 TBI Bought this truck a few months ago from a craigslist add. Truck was actually well maintained except there was a blown head gasket. Drove the truck home, popped off the heads, pressure tested them, cleaned em up, did a minor valve job and threw them back on using fel-pro gaskets and everything new. While I was in there, I replaced the water pump, t-stat and of course the usual tune up stuff. Fired right up BUT now I have the standard mystery temperature spike and then crazy fast drop. Thought it had to be some air bubbles...but It has been all crazy for a couple hundred miles now. Thought I should try a new t-stat, but i yanked mine, tested it and it tested good. Cooling system pressurizes, nothing leaks, no coolant being lost...I mean there is nothing happening here that leaves me to believe my repair is not good to go. Now there IS one dumb thing I haven't corrected yet and I am hoping that is it. When I threw the heads back on, well I flip flopped them. I was tired, at the time, and so I simply routed the temp sender wire around to the back of the passenger side head rather that switch the sender to the correct location (the plug was being a pain to get out and I was exhausted at the time). Soooooooooo...I am thinking that could likely be the cause, but I am uncertain if it would account for such radical temp variations. I currently have a 195 t-stat in there. Weird thing is the colder it gets, the hotter it gets on initial start up. Also I feel I should talk about the symptoms a little bit. If I let the car just run in the driveway, it will never fluctuate. If I drive it down the highway it is all over the place...tonight it ran up to 240-250 or so and then immediately shot back down to 190. I dont know...the ups and downs literally freak me out enough to ask people here for input. As you can see, I have already tried most of the common stuff. I keep reading over and over about similitude stories tho and no one ever posts up the results or any fixes...the topic just simply dies.