Open this beast back up and spray a can of compressed air to remove all the dust bunnies from the vent and the fan. Remove the fan and remove all the thermal paste with isopropyl alcohol and reapply with new thin coat of Arctic 5 Silver. Re-assemble and fire it back on. Reset PRAM by by pressing Command, Option, P, and R keys at same time until you hear the 3rd Bong sound. Then you can release the keys. Now go into disk utility, select your hard drive at the left of the screen, and click on repair permissions at bottom of screen. Also, reset SMC. See if that don't do the trick for ya bud.
When this beast is on put your ear toward the back vent to see if the fan kicks on, because your fan may be dead and not cooling the unit down.
Get the free utility SMC fan control online and use it to control your fan speed to cool this beast down.