Get rid of the digital. They don't keep up with the temp fluctuations very well. Heck, the little crappy ones that come with the incubator are probably better than a digital.
Check this unit out. This is what I used when I was using the styrofoam units....
http://www.cagenbird.com/Thermometers_9.htmSedcond one down = HIGH ACCURACY 6" THERMOMETER/HYGROMETER for $30. It works great in the styrofoam incubators....