Try changing the connection port to one between 49152 - 65535. A good one to try is 64049 for both listening & connection ports. I realise port 6346 proved unblocked but not necessarily shaped or filtered by potentially other sources such as isp.

I am curious why you'd have XP firewall disabled as well as running dmz. Aren't you worried about potential hazards that such security tries to block out? You are leaving your machine wide open for potential attacks. just IMHO

To be honest I'm not sure about how to advise someone using DMZ mode except perhaps they're comparable to dial up users in regards to no consideration of UPnP or Manual Port Forward.

AFAIK Comcast shouldn't give you problems. Are there any other computers networked with yours or using the same linksys device as a router?

Do you find you need to reboot your linksys from time to time or it just simply fails on you?

And are you doubly sure you're running in dmz mode? Did you save your settings for thus?