[mark_gober]

Does your blend door actually actuate? You could turn the engine one and turn it to cold. I'm sure it gets cold. Then slowly rotate the knob to hot. You should feel it getting hot. If it gets hot, I suspect the actuator works fine. I might be wrong on this, but I don't think these things usually fail "half way". When they fail, they fail.

You could also check the temp of your water hoses going to the heater core. They should be almost the exact same temp. If you have a clogged heater core, one will be considerably hotter than the other.

This is simply my unscientific opinion, but it sounds like your heater core is the more likely culprit. The heater core can supply X amount of heat. They are designed to be able to provide that much heat at the highest fan speed. If you reduce the amount of hot water flowing through it and keep the fan speed the same, you'll notice a reduction in temp. This is similar to A/C systems. If you take a perfectly good a/c and put a digital thermometer in the vent...it'll read a cold temp, say 44 degrees. If you then take the fan and reduce the speed, you'll see the temp go down. It actually is colder on slow speed than high speed. The slow speed temp will be around 4-6 degrees colder. This is because the air has longer time over the coils to transfer it's heat. In your case, you want to absorb heat, not reject it. Turning your fan up reduces the amount of time that the air is in contact with the already marginal coil. Slowing the speed makes the heat transfer more efficient and it feels warmer.

You could try to backflush the heater core. Just disconnect the hoses (both) and reverse the flow with a water hose set to low speed. LOW SPEED is important. If it's rusty and you hit it with high speed, you risk a nasty mess on the inside of the car.

Good luck and keep us posted.

Mark