Sorry, this won't help with your question, but interesting all the same:
My second car, a J reg MK 2 golf "Ryder" was a basic mule powered by a 1.3 8v Carb engine delivering a dizzy 54BHP. it really wouldn't pull the skin off a rice pudding! At the time I used to live in Holyhead, but used to travel home to Prestatyn on the A55, a journey of about 50 miles.
The car would get around halfway, then I would gradually lose power until even with my foot down all the way on the accelerator, I would not move.
I called the AA, when the man arrived it of course started straightaway, making me look a total chimp !
This happened again, so I pulled over and waited 5 mins, viola the car would start and drive no problem.
Anyways to cut a long story short I booked it into a local VW garage who without seeing the car a mechanic said "ah yes it will the throttle butterfly icing up" I believe they fitted some sort of shield, certainly this never happened again.
This would happen even on a fairly warm day, so don't discount this as the problem!