Weird LAN problem - wireless router degrades LAN performance

I'm suffering a strange LAN performance problem I can't figure out. I'm hoping somebody somewhere on the 'net might read this and offer a potential solution. Here goes....

I use Verizon FIOS for my internet connection. My service level is 20Mbps/5Mbps. My main PC is a alptop with a built-in 802.11n card. The card, according to Win7, is an Atheros AR9285.

Verizon FIOS supplied an ActionTec 802.11g router.

The wireless card in my laptop is DISABLED. The laptop is connected to the LAN using an ethernet cable, plugged in to the back of the ActionTec router.

Speed tests performed at http://www2.verizon.net/micro/speedtest/java/# consistently display results of 19.5+ Mbps / 6+ Mbps.

About a year ago I purchased a Belkin N150 router. I configured it as an Access Point and plugged it in to the back of the ActionTec router. I had found the ActionTec router to have unreliable wireless, and the Belkin's 802.11n would provide faster speeds and greater range.

A couple of weeks ago I started seeing frequent weird network drops. On my laptop network connectivity would seem to just stall every few minutes, although Windows diddn't  report that the connection had dropped. A few minutes later the connection would work again. If I disconnected the Belkin router from the network the problem disappeared.

I did a lot of troubleshooting, and ultimately found that if I used my laptop wired, with the wireless disabled, and disconnected the Belkin from the network, the problem disappeared. This would suggest the Belkin is broken somehow.

I've upgraded the Belkin firmware to the latest version. I've reset the Belkin to factory settings. Still, anytime the Belkin is on the LAN, my laptop gets terrible network performance. The speed test results drop dramatically, down as low as 0.54xx download speeds in some instances. The moment I disconnect the Belkin the network performance returns to 19.xxxMbps downloads.

There is something that makes the culprit less obvious, though. With the Belkin connected to the LAN, another laptop in the house sees no troubles at all, and still gets perfectly good network performance.

On my laptop I've reset the TCP/IP stack. I've deleted all entries from the ARP table. Of course I've rebooted multiple times. It just seems like my laptop is suddenly allergic to the Belkin router.

I'm stumped, and would really like to use the Belkin router if I can just resolve this.

Anybody out there have any idea what might be causing this?