Now thinking I may have a cylinder head or head gasket problem. It's losing coolant and I can't find a leak. I did a compression check when I first started having this problem and all cylinders were about 150 psi. No oil in coolant or coolant in oil but it is definitely loosing about 1/2 gallon every 200 miles or so. No smoke in exhaust. Is there a way I can test for this without taking it to a mechanic? I would like to be sure this is the problem before I pull the head. I just replaced the HG about 70,000 mi ago and put a nice rebuilt head on. Maybe I didn't do a very good job. Was thinking about re-torquing head bolts. Don't know if it would help at all.