I tried the troubleshooting steps you outlined.
1. Turning off Access Control made no difference.
2. I tried two other cat 5E patch cables and this made no difference.
3. I dug up an old Linksys router and turned off DCHP on the LAN. I inserted this device in between the cable modem and the PCWRT router so that it functioned as a switch. This does seem to resolve the issue, although the speeds I saw topped out at about 96mpbs (not surprising – it’s a 100 base T switch).
4. I borrowed a gigabit switch for testing and replaced the Linksys switch with the gigabit switch. The speeds are now as high as 260mbps when my PC is on the 5ghz wireless network and near the router.

Do these results indicate an interframe gap issue or something of that nature? Is there any way to fix this problem short of buying a redundant switch to sit permanently between the PCWRT router and the cable modem?