Looks like I need a new heater.... Any thoughts?

I just got a digital thermometer making it much easier to keep track of my water temp and now it looks like I need a new heater.  I initially thought it was kicking on when it shouldn't but didn't really dig into it any farther, just to lazy to get up and read the glass thermometer.  Now I have noticed the heater seems to have lost all its accuracy, it was set on 76 and my tank is 83.  Sitting on the couch watching I noticed it kick on even when the water temp at 83 so I cleaned the heater, removing all coraline and I bumped the temp down to 74.  I would guess I still need a new one. 
so....
Any suggestions?  Don't want to spend a ton, how about the heaters with the remote/digital displays?  This is for a 90 gallon and right now working on creating a reef tank out of it the old heater is a 300W Rena.