Overheating

You might want to have your mechanic perform a chemical test at the radiator to rule out a leaking cylinder head gasket, but since this does not occur at lower speeds, a good suspect is the radiator's cooling fins are corroded away. That limits the radiator's ability to give up its heat to the surrounding air. The clue is the problem is worse at higher speeds, and/or in higher ambient temperatures. You already used the additional clue of turning on the heater fan to help cool the engine.

Often when the cooling fins are corroded, they will crumble like a rotten chocolate chip cookie when you run your fingers over them.

Mine "looked" okay too when this happened to my minivan. In my case I also had a rear heater. Running just that one on the highest speed brought the engine temperature down within two minutes. I could not figure this out at first, until I started to have to patch holes. After about the seventh new hole in as many months, I finally got frustrated enough to install a used radiator with very low miles. Running too hot was solved. Only then did I figure out the fins were rotten.

You can also feel the top, middle, and bottom of the radiator when the engine is hot. If the bottom is almost as hot as the top, it' is not giving up its heat. Do not overlook a butterfly collection blocking air flow.

Other things to consider include there must be no holes in the core support that allow air to bypass the radiator, and there is a rubber seal that runs along the front edge of the hood for the same reason. Be sure that is still there.