Whoaa... so why was the thermistor based temperature sensor not cutting it for you ?

Did the stove eventually respond to the thermostat? It's hard to diagnose without more details, but here are a few things to check:
1) Does the thermostat work at all? If you set the temp down to it's lowest setting does it turn the stove off? If not it is either defective or miswired.
2) Check the wiring. Often these thermostats are for heating & cooling, so there is a jumper needed for the common (RH to RC). Look at the directions again to make sure it is wired correctly. Be sure it is connected to the heating terminals and not the cooling terminals (W and RH).

3. If the thermostat does respond, albeit slowly, then you may need to adjust the differential or temperature swing. This affects the number of degrees between the on and off operation. Normally instructions for this are under the advanced features section. You want the differential set so that the thermostat is responsive, but not overly sensitive. If it is too sensitive, the stove will short-cycle (turn on and off too frequently). If it is too insensitive, there will be 2+ degrees swings in temperature before the stove responds.
   4) Is the thermostat placed away from drafts, but in a place where it can respond to the heat of the stove in the room (area), but not be directly affected by the stove?

The Lux thermostat was real cheap (bi-metal) seemed like there was too much between the turn on and turn off played with the differential/temperature swing to no avail, Went with a honeywell and it is much more accurate (mercury bulb). As far as the digitals go when I hooked them up I tried every possible wiring method for all the thermostats and what would happen as soon as I hooked it up is that the thermostat acted like it was calling for heat no matter how low it was set, Yet if I tied it to my furnace it worked fine. I think the voltage supplied from the Harmon circuit board is a low DC voltage and the diodes in the thermostat were stopping the function of it since the thermostats are designed for AC voltage. Never did throw my VOM at it just went with the honeywell...

Ok Finally solved this, I have a friend that has his own heating/AC business and he tried a LUXPRO PSD111 digital thermostat and it works fine with the harman circuit is pretty accurate also .5 to .25 temp swing 1 deg accuracy backlit and seems well made...

You hooked the thermostat to the heat sensor leads? I have a P68 will it work here rather than the sensor which came with the stove?

No, on the P38 there is no sensor so all you can do is use a thermostat in place of the sensor. I don't think that you can hook a thermostat to the P68...

I had a similar problem with a P38+ installed in '07 that we managed to solve with the help of the dealer and an electronics engineer from Harman. The P38 uses a different electronic control board than the other stoves and does NOT use a milivolt thermostat. On the P38 _ONLY_ the output to the thermostat is a combination of 60V AC and 50V DC. This is apparently normal. As a result, the internal surge protectors of better built 24V max thermostats (multilayer varistors, MLV's in the case of the Honeywell I have) short circuit the stove to protect the thermostat from an apparent overload voltage. From the stove's point of view this is the same as the thermostat calling for heat, so it goes into turbo mode, as it should. To solve this problem you need a thermostat that is rated to 60V DC (nearly impossible to find), find a cheap thermostat that has no internal surge protectors, or remove the surge protectors from a thermostat (at your own risk, of course).

do any one know where i can find a herman p38 pellet stove control board