heat works only at city speeds

[TurboSDL]

so i took my car for a 240 mile trip yesterday and when im around town the heat works good but once i get on the hwy the heat blows cool, not super cold but not warm either and the temp gauge reads abt 80 so i was wondering if anyone had any idea what could be the issue... also my 2 middle vents on the dash wont blow any air out either, the vents open but no matter what i select on the selector panel the air only blows out of the defrost and the 2 side vents. any ideas would be greatly appreciated.