Most probably it is due to vapor lock.
You know why the instructor told you to NEVER turn the engine off at the traffic signal? Because the cooling system would also stop and, for a brief period of time, the engine would go higher than the favorable temperature of combustion (so you have a difficulty starting and have to crank a lot).
However when the engine cools down to a more favorable temperature, starting is a breeze.
In highway traveling, it happens the most in Asr prayer. The prayer in itself is small, and like Zuhr you don't have to eat lunch. Hence when you come back after 6-10 minutes, and the engine won't start, you'd think something's wrong. I remember I used to freak out when as a kid I was accompanying my dad on his journeys. (The sun is setting down and it's the universal time of depression, you are tired and almost feel lost).
The engine is more stressed at high speeds (and for longer periods), that's why the problem only occurs at high speeds. The effect can be minimized with further tuning. Also check out if you're traveling in the direction of wind or against it.