Sounds like you're using the Tetra submersible heater. I had that one at least and it did to me what yours is doing to you now. =/
DUDE. You hadn't written it yet and I already knew! XD That heater is the pits. it's suitable for cycling but not much else IMO.
Get a Hydor Theo or a Marineland Stealth heater. They're adjustable and as long as you leave the shaft underwater it should be fine.
Since I got my marineland stealth heater the temps been at a constant 79*. =] I'm very happy with it. =]
Just avoid the marineland stealth PRO. The Pro explodes. The normal stealth is the only one sold in stores though I think. Plus a good look at the packaging will tell you if it's a Pro or not as it'd be in the name. =]
Good luck with your little guy! =]