Well first of all, the factory sensor may not have the same characteristics when it changes its resistance with temperature change. Also the sensor my be bad.

I would say flush out your heater core again. Do it in reverse flow then forward flow. I had to take mine to a do it your self car wash. Must more pressure. But you do run the chance of blowing the heater core and coolant running inside the cab. But it did the trick. I ran the washer through the core for about a good 5 mins. I wanted it clean.

 

heres one no ones touched on....which way did you put the t-stat in, did you put it in backwards by chance?

 

i put the thermostat in right and i think i am just goin to buy a new heater core and replace it and hopefullly that does it