If your heater is just plugged in to the wall, the bottom of your tank is probably pretty hot. Its good to take a reading at the bottom, because when they burrow they could get burned if its too hot. Once you can connect a rheostat, try thinning the substrate down to a minimum. I use paper towel underneath my substrate, with a thin layer (half inch of aspen)on top, on the warm side. I keep my glass temp at about 88 degrees and it keeps my substrate at about 80-84. I was using quite a thick layer of aspen before, and i found that at the glass my temps were into the 100's before i could get the top of my substrate close to 80. What is cob, by the way? Never heard of it.
Josh