You may want to invest in a higher quality heater. The other thing you could do is use a controller of some sort. They can turn it on and off based on the readings of the temp probe it comes with. They are usually much more accurate. I like the Eheim heaters and the Cobalt Neotherm.
Try an experiment, set it at 76 and see if it still jumps up to 80-82, your problem might not be the heater. You might need to put a small computer fan blowing on the water to help cool.
Heat can come from many sources....pumps, lights, power heads. Maybe turn it down a couple degrees and see if it still jumps.
Getting a controller may be the right decision, but maybe a better quality heater could be the fix too.
A small fan??? Well I have three buttons on the tank each with its a own lighting when I hit one of them the lighting is suttle and it seems to kick on some kind of fan