Hey DG, welcome to the discussion. It reads like you've done some homework on this subject and tried a few things but have been unsuccessful. Here are a couple of other suggestions:
Shorten the 802.11 broadcast spectrum. I've run into quite a few of these routers that seem to have an issue broadcasting a mixed mode WLAN signal. I'd recommend logging into the Management and Diagnostic Console http://192.168.1.254/mdc and clicking "wireless" on the left under "local network". Then scroll down to the wireless broadcast mode and see if you're broadcasting in 802.11b/g mode. If so, change the mode to 802.11g ONLY and save the setting. Monitor the connection for a while and see if you get some relief. If it continues, log back in and try it with 802.11b ONLY for a couple days. People have raised the argument that reduces the throughput of the wireless at 802.11b but the max transfer rate for "b" wireless is 11Mbps. Your Internet connection for DSL is 6Mbps so unless you're streaming movies from one local machine to another, you shouldn't really notice any restriction. Let us know how it goes and good luck.