Does your Linksys have bridge capability?

You may make the Linksys Router to be transparent or bridged, with the DHCP disabled, it would help faster after the modem through the linksys.

There is another way: Disable the Modem's wireless, but keep the Linksys's wireless and DHCP, ie. you bypass the router functionality of the modem, let the Linksys handle the PC's IP. That is how many are ding, pretty good speed control, little loss.