i had a similar problem. found the thermostat was sticking. also i found out if i filled the antirfeeze tank to the full mark of an inch overfull, that expansion would make the tank go over pressure and cap leak. i now try to keep the level slightly under the cold full mark. have had much less problems. NO problems for first 3 years, no for some reason i have the high temp for 30 seconds then drops back to normal. New thermostat and slightly lower level has helped quite a bit.