What size UTH do you have, where did you place the thermostat probe, what are you using for substrate, and what is the bulb wattage?

Ideally you should place the thermostat probe somewhere static... say taped directly to the UTH. You could possibly plug both the UTH and bulb into the Herpstat, but you may have problems. I'd suggest running it past Dion from Spyder Robotics (they make Herpstat) to see if there's a problem.

Also, what are the room temperatures? A small UTH and low watt bulb might have a hard time heating a 20 gallon tank in a cold room.