Do you have any extra fans cooling your computer?
Are you overclocking your vid card?
Do you have dial up or broadband?
Could be overheating, you should invest in extra fans. I recommend the Vantec fan card for your video card and better heatsink/fan for your CPU.
If you have dial up, you will get booted from online gaming all the time. When I used to play EQ online with dial up the same happened to me.
Also, check the seating of your vid card, ram, and cables. Doom 3 would go out on me all the time because I had a cheap hard drive cable.
Let us know,
Your Hardware Guide.