i adjust my heater to 70, still high it stays at 84-86. i
Is it the correct wattage for teh tank size? I've never found the temp settings on heaters to be that accurate, though my stealth heater is pretty close. Always use a good thermometer and adjust the heater according to that. If you feel the heater may be malfunctioning or at the end of it's lifespan, replace it. Many times heaters stick ON when they go bad, and can fry a tank.
I'm not sure about your lighting (* filling out the "my info" button will help in the future) but I found it necessary to put my heaters on timers, so they are shut off when my lights come on. They kept driving up temps. Many times fluorescent lighting will not produce enough heat to maintain tank temps by themselves though. I have MH
so it's not an issue for me.