In addition to things already mentioned, there are two components that could be the culprits in your problem, and they're both part of the emissions system.

You probably have an "oxygen sensor" in the exhaust manifold that tells the engine control (call it the computer if you must) when the mixture is too rich. At cruise, the controller tries to run as lean as possible without causing excessive combustion temperatures. If the sensor fails, it screws up the mixture control and it may go lean and cause overheating. If you get to the point of having a "professional" work on your car, DON'T LET HIM TOUCH THE CARBURETOR until he has TESTED the oxy sensor - or proved that you don't have one. Scream and holler and make threats, if necessary.

Many autos from your vintage and later also have one or more "knock sensors." Their function is to signal the "computer" to retard the spark and slightly enrich the mixture if it goes lean enough to knock. It should work at "subaudible" levels of knock, so you won't hear it, but if the knock sensor(s) go bad your mixture goes haywire.

(The knock sensors are the most frequent cause of the "check engine" idiot light that lights up at cruise speed. If the knock exceeds a certain threshold, the light comes on. It goes out if you shut the engine off and restart. And for some reason, the computer "memory" doesn't retain why the light came on. Us a higher octane fuel and the problem goes away. Most common in "larger" vehicles than the one in question.)

EITHER of these components, when present, can be totally destroyed by even small amounts of leaded or otherwise contaminated fuel, although it's usually the oxy sensor that goes. Much is said about leaded fuel poisoning your catalytic converter, and it will, but a malfunctioning converter usually doesn't have much effect on engine performance; but loss of the oxy sensor and/or knock sensors will prevent you from ever getting things otherwise "right."

John