27in 2010 [Mid] Running Really Hot During Game Play

Hello,

I've seen a couple other concerned users in here and figured I'd post my issue with my Oct (mid) 2010 27in i5 iMac.

For starters:

2.8ghz Intel i5

16gb of 1333 ddr3

ATI Radeon HD 5759 1024mb

Mac OS Lion 10.7.4

During game play the temperature of the top portion of the iMac exceeds 120'F, I'm fine with the heat, It's a large sealed aluminum case housing lots of hot items. Here's the catch my graphics begin to deteriorate and pixel/hexigon away so gaming is no longer visible... Now I've got a desk fan running behind the computer and can get a few more minutes before the pixel/hexigonal phase hits and game play must stop or be tolerated.

I just downloaded iStat widget to attempt to capture some temperature and fan data. Could this issue not be the temperature and maybe my GPU? Is there a standard limit of temperature to determine maybe my fans are not functioning correctly? Is there a safe way to clean the iMac's innerds without damaging anything?

Thanks -

(1) At Idle

(2) In game

(3) Shut down from game and attempting to cool down

Here's a snap shot of the graphics deteriorating, eventually the entire screen fills with "lack of graphics"

*Game= Civ5 within Steam (for Mac)