Heater Trouble

Now that I fixed my heater controls, once the car gets warmed up it's nice and toasty.
But once I get on the Interstate, if I decide to haul some ass and wind it up around 5,000 rpms in any gear, all of a sudden the heater starts to blow cold air.
Sometimes winding it up again makes the warm air come back. I watch the temperature guage, and it's not overheating (or underheating).
Any ideas what I might check? Anyone else had this one?
Thanks.
Flunky