I have a bizzare phenomenon happening in the cab heating system. (85-300TD W123)
I have the radiator covered for the winter to keep things warmer in our winter months (especially with a veg-oil system). When I am driving on a highway and get to just over 100km/hr (60mph) or so the heat stops coming out of the vents and the engine temp rises fairly quickly. The fan continues to blow however.

I have to decelerate to less than 80KM/hr (50mph) in order for it to switch back to blowing hot air and decrease the engine temp. Would there be a particular flap in question that can be oiled or something?

I could also use more heat period. I cant seem to gte a nice heat coming out and when the car is idling its almost cold. I need to bring up th rpms to get heat pumping through better. Any ideas?
Thanks