I'd say air is getting into the coolant system. I'd make a small-1/8'th inch hole into the outer ring of the thermostat valve so that the coolant system doesn't have to endure air-pockets forming during cool-off cycles. This simple modification can erase years of factory engineering designed to cause a major breakdown,at a predetermined mileage, that is very success full at separating you and others, from a huge amount of cash to fix. My idea is to keep the coolant level honestly topped-off regardless of temp and running or not. WARNING-NEVER take a radiator cap off on an engine that is fully warmed-up! ALLWAYS allow it an hour or two to cool-off first! Hope this helps.