Sounds like it's airlocked. Does it boil over too? Drill a small by-pass hole into the flange of the thermostat to let the trapped air out from under the t-stat and park it nose up on a steep incline. This purges the air out every time for me. Also, when it "burps" the air out, shut it off for a few minutes, add some coolant and start it again. Feel around for cool spots like the lower radiator. They tend to plug up with age,
Good luck,
Scott
