When outside temp is very cold, the heat comes on fine. After about ten to twenty minutes of highway driving, however, only cold air blows. Sometimes I can get it hot again by turning the temp dial to off position and returning it to heat. Other times it just comes back when it wants to... Two recent times, with the heat on full, only cold air came out of the vents, and the temperature gage's needle went to bump up against the red zone, indicating an overheating situation. I thought that maybe my antifreeze was low -- and therefore, causing the crisis above -- so I topped the reserve tank off to the max line. Two recent times, this seemed to help for a day or two. Could something be blocked? Any comments or suggestions please? Thanks in advance.

P.S.: Does it matter if the air recyle button is on/off?