That's perfectly normal, that's just the thermastadt turning the heater off when it has reached the desired temp, and when the temp drops below that it turns back on again. You don't have to buy a new heater that's how a heater works
I bought a small Whisper heater for 5-15 gal. tanks, for my 5 gal. tank. It's just like the one I have in my ten gal. tank except that it's a preset heater that keeps the temp. at 78 degrees. It doesn't flicker on and off like the bigger heater does. I like that better.