Ive never used the All Glass Aquarium heaters, Ive had good luck with Hagen Tronic heaters an Visa-therm heaters. Im trying a new titainium heater now.
But it sounds like you got a slightly faulty heater.
If it used to work fine and now is starting to drift then you best start shopping for a good quality electronic type. Heaters are a critical piece of equipment. You should always purchase the best quality you can afford.
The one you have is most likley controlled by a bi-metalic strip that is starting to work harden, Eventually it will turn on one day and not turn off.
You can imagine the rest.