If the car waws sitting in a humid climate (lots-0-rain etc) then water will condence in the engine. After you run it it turns to vapor and migrates to the valve cover area and accumulates when it gets cold. Change the oil and filter, run it for about 50 miles at a strech then change both again. If you have the aftermarket cracnkcase breather change it too or take it off and dry with a hair dryer as that is where the vapor will want to go. This is easier and cheaper than just replace ing a head gasket. Hopefully this works, if not keep me posted anyway- Good Luck!