Hoping someone out here has had experience diagnosing this problem.
My '74 450SLC (with approx. 100K on the clock) has a stalling problem after a prolonged run on the highway in hot weather. The engine is NOT overheating, temperatures generally peaking no higher than 110ºC and returning to between 90-100º pretty quickly after exiting the highway. After such a run however, the engine stalls at stoplights when in drive. When in park or neutral, the idle speed is around 850-900 rpm as usual, but dies when engaging drive unless I apply throttle.

The problem does not manifest itself in regular in-town driving (mixed street/hwy). It takes a prolonged run in the highway in 90+ºF heat. The car generally runs well otherwise, just a little shake at idle, especially when cold.

My local mechanic has checked and adjusted the CO at idle (ECU) and load (MAP). I have disassembled and cleaned the auxiliary air valve which was sticky. It now is open when cold and fully closed at around 80ºC. The spark plugs (heat range 9) are in good shape and have a very light tan color on the electrode insulators.

Could this be vapor lock? If so, would adjusting the fuel pressure regulator be a good next step?

Or: bad temperature sensor(s), vacuum leaks, valve clearances too tight? I really don't want to take a shotgun approach to solving this!