Hot then Cold

So, it's 40 degrees outside, I have the Heat Temp Control up to Full Hot, and I drive around town, it gets plenty toasty in the cabin and the vents blow very hot air.

Yet when I am crusing down the highway for just a minutue or two, the air becomes as cold as the outside air and I freeze my butt off!

What's this all about?!?

I am taking a stab at this, but I am thinking a vacuum leak. Do the headlights come up Ok at high way speed? If not then that is the problem. I know under exceleration you will lose vacuum and if ther is a leak you will lose it out of the container that holds the extra vacuum. This in turn might close one of the vents that is being held open by this vacuum. This is just a guess. If this turns out to be then the first place I would look is the the check valve in the engine compartment that keeps the vacuum from going back into the engine. Also, check to see if you can raise the headlights after the engine is off for about 20 seconds. That also is the sign of a leak.

Or you can check the anti freeze level in the radiator.
If it's low, then it will starve the heater core for hot anti freeze at higher speeds/rpm's.
I had this happen to me on my '76 Regal years ago. Every time the heater blew cold air, I knew it was time to top off my leaky radiator. And this always happened at highway speeds or high rev's.
Good Luck!
Sarge