GWguy
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More likely, it could be caused by the pistons pushing air into the water system thru a crack in the head/manifold/head gasket. I seem to remember this happening to a company Ford Escort we had with an in-line transverse mounted 6. I'll ask my office guy tomorrow what the resolution was, if he remembers.
River, I talked with our office manager. Our Taurus had weird problems, like you could hear gurgling in the heater core, the temp gauge would jump sky high and then come back down again... turned out to be a blown head gasket. It was allowing air into the water jacket. When the gauge jumped up, it was because the temp probe was in steam, not water.
I'd look for signs of a blown head gasket. Look for signs of contamination in the radiator. Check the oil for signs of water (usually a tan to brown sludge). Use a radiator pressure tester to see if the system holds pressure.
Best I can offer without seeing it. Hope that helps.