i had that happen on my 1978 GMC pickup, and i found out that the radiator cap was on too tight and didn't let it breathe..........i had to replace it and now it is fine. i also replaced the coolant in it. hope this helps you on your quest. 
1949 Dodge Coronet 1955 2nd Chevy 4400 1.5 ton 1955 2nd Chevy 3100 1/2 ton 1955 2nd Chevy 3100 1/2 ton 1957 Chevy 5400 LCF 2 ton 1966 Dodge D100 Sweptline 1968 Chevy P20 stepvan 1969 GMC LWB pickup 1972 GMC Sprint 1974 CP30 shorty bus There are three things that I've learned never discuss with people: religion, politics, and the Great Pumpkin.----Linus Van Pelt Trying to understand the behavior of some people is like trying to smell the color 9  | 
