remove both heater core hoses at the engine. blow throw there with a garden hose and nozzle on the end. Flush the core, back and forth so it's clear both ways. Be prepared to turn your temp down :)
I have a thread on here about the car overheating. In short, It's got new heater hoses, and I flushed the whole system MANY times. I do not believe it to be a coolant flow issue, although I wont discount that thought. Car does not have A/C currently and also the flaps inside the heater box are all working.
It actually sounds like there isn't much resistance at the fan.. I'm going to pull it and see if there is a bunch of blades missing.