Have you had any problems with loosing coolant but no visable leaks? Mine would miss for about 10- 20 seconds when I first started it in the morning. It turned out to be the result of a leaking headgasket. The coolant would leak into the cylinder while parked and foul the plug. After the coolant was burned out of the cylinder it would run fine. A sure way to spot this condition is to pull the plugs and compare them with the other two on each side. The cylinder that is leaking will have a clean white center electrode on the plug as compared to the other two cylinders on that side. It is usually the rear cylinder that leaks. Here is a link to how I resolved this problem.
Looks like I'm headed down the "Headgasket Highway"....again