I have never seen a DoS attack on a PC during a game. I have delt with Hacks, and other such problems during games but not a DoS attack. Possible, I suppose. What port does the game use? Is there another acceptable port for the game? I think that is how is getting in. Your firewall is disregarding packets sent through that port.
I have heard of Packet Sniffers ETC. While I am no expert on networking security, from what I hear, they are not very effective.
But I would see if the game can use another port, and Contact the ISP abuse line If this really is a Dos attack, you need to put an end to it.
Sorry I can not be of more Help
Sean