It looks like it is coming out through
the radiator, but not all the time! Could
that be from a bad thermostat?
No. The thermostat will not cause the radiator to leak, unless a stuck thermostat causes overheating and the over-pressure will come out the path of least resistance, preferably out the cap in to the overflow tank when the pressure compresses the spring on the cap. Buy/borrow/rent/steal (just kidding on that one) a pressure tester that you put on in place of the cap, fill radiator then pump up with air until you see where leak is. Do this with engine cold.
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