Over the past 3 days, I've noticed that when I have the heat on in my 2004 Jeep Liberty, it blows cold air. It seems like when I'm on the highway and have the RPM's up, it blows warm air. Once back on the side roads or idling, it starts blowing cold air again.

You may need to reverse flush the heater core and cooling system, due to corrosion or phosphate scale. Tap and well water are loaded with minerals that can build-up and block radiator and heating cores. Minerals found in tap water tend to combine with coolant additives. When this happens, they form a chemical complex called “phosphate scale,” which coats the heat-transfer surfaces of radiators, heater cores, and heads, resulting in decreased heat-transfer efficiency. Refill with G-05 (HOAT) antifreeze and distilled water.