This is just for your info.
I was having almost the same exact problem with iChat (and other .Mac programs) with my Linksys router. I tried numerous firmwares, port openings and complaining on the Linksys router. Well, after reading over MANY Apple discussions and other internet boards I came to the conclusion that Linksys and Apple DO NOT LIKE to work together! An Apple moderator noticed that MOST iChat problems involved Linksys router. Even when the router has all the right port openings! So, I tried just using Apple software firewall and iChat (and other .Mac programs) worked flawlessly!. So, read on these internet boards and Apple discussion boards. I noticed that people with other brands of routers were NOT making the disconnect and drop complaints! I then went out, bought a D-Link router, installed it and I have NEVER seen anymore problems with the iChat no connects and drop connects!!!
Now here are my PERSONAL observations about Apple software and Linksys routers.
1) iChat will only work properly (cross your fingers thought) if you use the firmware of 1.44.2 and open the ports that
Apple suggests in the router (firmware 1.44.2 work the best IMHO). However, the .Mac software for the software"Backup" will NOT work with ANY Linksys router!
2) IMHO get rid of the Linksys router in you equation. Try it, and you will see iChat will work flawlessly. I use another brand of router(D-Link but other brands are just as good)and I have had ZERO problems (unless a friend is using a Linksys router ::evil:: ).
This is just IMHO. Good Luck!