I have this same problem. Same router as you. Comcast ISP. I fixed it by

Tools > Options > +Advanced > Firewall Config > Under Router Configuration, check Do Nothing box > restart limewire

However my problem with this is that it seems to lessen average download speed. While being able to download for as long as i want and surf the web is better than only being able to download for about 5 - 15 minutes at a time and have no http access, I would prefer being able to download at the speeds I know my internet is capable of.