I have fixed a similar problem. I suspect that the impeller blades on the water pump are eroded but flush the heater core first. Last year, my mother-in-law's 1999 Taurus had been blowing cold air for a while but it wasn't overheating. I flushed out the heater core as best I could using garden hose. The flow through the heater core didn't seem clogged. I pinched off the bypass hose on the engine side of the firewall during the winter and removed the pinch when the weather got warm (not recommended). This year, her heat got worse so I replaced the water pump for about $32 and a few hours labor (I like to take my time). Here is a picture of the impeller blades .
There's a larger pic on
http://home.comcast.net/~luthienkona/howto/fordheat.html
- george