I've read the only post I could find on the Shaw forums https://community.shaw.ca/thread/5001 referring to the same problem I have. In the case of some online games (particularly the multi-player content of Battlefield 3), the issue manifests itself as a "rubberband" effect, where my avatar is apparently in the wrong place, and the game attempts to re-adjust where my avatar "should" be. This causes my avatar to run in a particular spot, or "teleport" around at random, or causes certain commands to not register. Sometimes during these events, the ping becomes very high.
I've determined the problem only seems to manifest itself when there is more than one user online in the house. So I can only infer the there is some sort of interference which causes packet loss in the router. I have the same issue when the game port settings in the router have both been enabled and disabled. As suggested in the other post, I've run a Speedtest, and my ping is 5ms with an approx download speed of 48Mbps, and an upload speed of approx 3Mbps, so everything on that end looks fine.
Under the Security settings in the router, I have also check-marked "Block Fragmented IP Packets", "Block Port Scan Detection", and "IP Flood Detection". This seems to help slightly, but the effect still persists.
Any other suggestions?