Agree with AMD... keep the heater, but spring for a controller. Either the Ranco linked to, or a Johnson Controls A419. The bi-metalic thermostats in the majority of these heaters are just not accurate or reliable enough once you start getting some $ invested in a tank. Using a solid-state controller that reads the temperature and decides whether or not to supply power to the heater is the most reliable way to go.
I always used to futz with my heaters - even with two of them - every now and then before I got my controller. I've never had to mess with them since.
Yup... using a controller is easy. Just put the temperature probe somewhere that is a good representation of the whole tank temperature and plug the heaters into the controller. Set the controller's high and low setpoints and you're good to go. You'll need to turn the thermostat on your heaters up so that the thermostat in them don't override the controller. But it's a good idea not to turn them up all the way... set them for something higher than normal, but not boiling. That way, if something goes wrong with the controller, your thermostats in the heaters act as a secondary safety switch to cut them off if things get to hot.