i would suspect that your thermostat is going bad. Its easy to remove, test and replace if necessary. First let you car cool down, this is a very important step. I like to drain some of the antfreeze from my radiator beforehand to make less of a mess. THere is a small plastic drain-pug underneath the radiator.
the thermostat cover is behind the distributor. You can follow you lower radiator hose back to the engine block and you will see it. remove the two bolts to remove the thermostat cover and expose the thermostat itself. remove the rubber gasket and test like this..
I hope this helps
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