Overheating Problems

I have a 2000 Ford Explorer with a 4.0 motor and 121,000 miles. I had the head replaced in April 2006. My check engine light came on and they told me it was the 6th cylinder misfire. I had it check and I had a cracked spark plug. The car was sputtering when I was driving. Could that have been because of the spark plug? The car overheated on me once so the mechanic thought it was the head again. They put silver soder in to seal the crack if there was one and they also installed a new thermostat (a 185 instead of a 195) to keep it from overheating. I seems to be loosing water also. I am not sure where it is going but I am having to put water in the overflow tank often. I took it to another mechanic and they did a pressure test and could not find a leak anywhere. The car is trying to overheat again. I have water in the overflow tank. Do you have any suggestions?

I had a cracked spark plug. The car was sputtering when I was driving. Could that have been because of the spark plug? The car overheated on me once so the mechanic thought it was the head again. They put silver soder in to seal the crack…

Was that solder used in the spark plug or the head?