My 850 wagon 94 model is overheating,and thanks to some of the post on this sight and my Haynes manual I was able to replace the Headgasket and fab the pull down tools for the upper camshaft and get it running again but now....  it has been a month, and now it is overheating again. It will sit there and idle and stay cool for at least an hour and a half maybe forever but, as soon as I get out on the road it overheats again. The coolant system seems to be overpressurizing too, to the point that it caused the coolant recover cap to break. I don't seem to be lossing water.  I think the the piston compression is blowing by into the coolant system causing the pressure. I have put the water hose to every hose section on the engine and the water always passes it through so I am assuming the engine is not stopped up and water flows in and out of the raditor.