HDTV leads to overheating?

My rig is listed below and this issue is related to this box. I have my 9800 going to 2 monitors (both DVI out). One is an old Dell 20" CRT. The other is my new 47" LG HDTV. Here is the issue:

When playing Crysis/ COD MW2, etc max/ close to max setting on the 47" I will play for an hour or so and then -blip- off goes my PC. I can turn it right back on and swap it over to the 20" on higher resolution max setting and not have a problem. It is obvioulsy overheating in responce to playing on the large Monitor so my questions is do people think its my Vid card or CPU? I would guess my Vid card as my CPU has a money heatsink and plenty of circulation.

If it is my GPU would it help to get a second 9800 for SLI or just go with a new nvidea 275 or so? Or should I not even try and play on my HDTV (please don't tell me this)

Or if you have any other theories of why this is happing and any thoughs it would be great.

Thanks!

Sig not showing up so here is my rig:

EVGA NFORCE 680I SLI 775 MB
Q6600 2.4GHZ
9800 GTX+
74GB Raptor for OS
1 TB for Data
4 GB Ram
850 Watts of power
Win 7 64bit