Where exactly is the herpstat probe? it is very common that flexwatt has temperature variation edge to edge and from the end to end. What is the power output on the herpstat reading % wise it may be the probe if it is under has pulled free or is being effected by something else. Do you have aluminum tape on the probe? This can cause false readings. The last thing is are all the accurites reading the exact same if placed in close contact? I have never found them to be very accurate and probes can easily be damaged. The way digital thermometers work is to measure micro changes in resistance, if the wire is damages at all it can change this by a large degree. (I have one that had a door closed on it hard and it changed by 18ºF)