The heater or the temperature of the water, if it's the water then I'm thinking the MH lights are heating it up, if it is then you might want to buy some small computer fans and install them under the light to keep it cool. what type of heater do you have now?
Its a titanium submersible heater. The sensor the tells the dial what the temp is of the water gets messed up from the MH. When I turn on the MH the heater goes red and stays on. The temp goes higher and higher where I have to turn down the heaterr. Once the MH is off the light on the dial goes off.
Have you tried moving the heater all the way to the bottom of the tank? 300watts is way over kill. I have a 150w heater in my 29 gallon and I have to keep it at 74 to heat my tank at 80 since it's so powerful. If moving it down doesn't work I don't know what's wrong. You can try buying a new heater and if that doesn't work then return it and you'l have to figure something out. that's an excellent heater with a lifetime warranty.