I've run across this some problem with under tank heaters. For some reason, they don't seem to get back up to speed after they've been shut off.
I run a UTH on one side of my 29 gallon tank, with a 150 watt ceramic up above (on the same side of the tank) Both of these heating devices are controlled by the same thermostat. The probe for the thermostat is placed directly on the substrate, inside the "hot hide". (I've also got a flourescent bulb above the tank, but it is not a heat source.)
In addition, I have two digital thermometers, one on either side of the tank, placed directly on the substrate. On a very cold morning, I can get the hot side to read about 82 degrees, whereas the cold side is about 75 degrees. Once the house warms up, the thermostat cuts off the devices so's I don't cook my snakes.
Recently, since I've noticed the performance problems with the UTH units, I've taken them off of the thermostat and now have them plugged directly into the wall. The ceramic is still temperature controlled, but the UTH are always on.
Some mornings, I notice that my adult snow climbs up on a high acrylic shelf inside the tank and is basking right out in the open, about 4" from the ceramic. (screen protected)
I'm no expert, but this is what works for me.

