Hey quick question...Follow

Just recently I moved my computer under a table next to the wall. (no vents are covered by the wall) Everytime I play any MMORPG or high graphics game my computer will just turn off. I checked the heat and the processor was at 81 C I believe. So it's running hot. I took off the side panel and reset to defaults on my BIOs. Now it seems to be running GREAT not locking up or anything but, it's sitting at 100% processes when sitting idle. I'll pick up on something and be completely lagged out for about 5 minutes then it will pick RIGHT up and run fine. Are any of these things related is my question I guess... and what might be causing this.

Yeah it's near a heat vent but we haven't used them in YEARS (And I mean directly on it) Hmmm now that I think about it was having the same problem when we first moved into the house.