This might sound goofy, and probably belongs in the "biggest blunders" post, but could you have put any diesel fuel in your tank? I drove my '72 Camino from Chicago to Greensboro a couple years ago. On the last leg of the trip, I swore I blew a head gasket or something. White smoke would billow out every time I accelerated, and it sounded terrible, but the guages showed all temps and pressures up to snuff, so I pushed on. After getting there and probing the yellow pages for a reliable garage, I found one. They discovered I had put about 8 gallons of diesel in at the last fuel stop. (In W Virginia) DUH!! They took down the tank, flushed everything out, and the car was fine after that. I was a bit under the weather on the trip, and had never run across a station that had diesel at the same bank of pumps as all the rest. They're usually far away from the other pumps. Guess I just wasn't payin' attention.
But now I look before I fuel.