I'm an avid gamer myself so I can understand your frustration. I just can't see any cause or explanation for your issues as they're not showing up on standard tests. Maybe some other people will have more insightful ideas?
I've got a few more suggestions here:
1) Turn off IP flood detection and leave it off. It's bad. Turn off the firewall too if you're willing.
2) Post the modem power levels and logs
3) Do a couple speed tests at speedtest.net, London server, and post the results, post the results of your pingtests as well while you're at it
4) Do a full analysis at
http://netalyzr.icsi.berkeley.edu/ and post the results (remove your IP address if you want)