Loosing Coolant
 

OK, here is a stumper to me...
I have a 1998 F150 with the 4.6L engine. 71,500 miles. Recently I noticed that after driving it approx 5 miles, the pressure cap would release and it would spill out just enough coolant to notice and smell. After the second occurance, I drained, flushed, and re-filled the system. I also replaced the cap with an OEM one. Seemed to solve the problem until about a month later. I have no cross contamination into other fluids, still have heat, temp guage still reads as it always has, and the engine has NEVER overheated. I have a theory as to possibly having a head gasket with either a crack or small hole to the cylinder and that as the piston is coming up on its compression stroke a small amout of pressure is entering the cooling system and over time this builds up to enough to cause the cap to max out and release. The engine runs just as it always has, and I have let it sit overnight and let someone else start it while me being by exhaust and have never smelled coolant upon start-up. Is this possible or am I overlooking something else?? Any info would be GREATLY appreciated!!