Most likely your thermostat is stuck closed causing the fluid to only circulate in the engine and not through tha rad, the same thing happened to my 91 ford ranger, it is an easy fix as long as you follow the directions in a hanes/chilton/whatever manual and on the package the thermostat comes in.. Manual should cost around $30 and a thermostat is cheap cheap, mine was $10 probably a little more for a volvo. needs few tools and took me about 15 minutes.
I wonder if the 1976 models have the compensation board problems. Do you actually hear the fluid boiling?
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