2006 iMac weak/lost Wi-Fi signal

My late 2006, 24" iMac (OS X Lion) is unable to load even the simplest web pages via Wi-Fi even though it appears to have a full-strength signal. On start-up, the browser will open a few pages, but within a couple of minutes, pages will begin to stall, and the browser tab will display a endless "Connecting..." message. Meanwhile, the Wi-Fi status menu displays a full-strength signal. I have had the Airport card recently replaced in the hopes of solving the issue, and this did not help. I have upgraded the OS from Snow Leopard to Lion, and this did not help. I have tried simple things like resetting the PRAM and creating a new Network Location. What's even more frustrating is that all my other laptops and OS devices connect just fine to the same Wi-Fi connection, and until a few months ago, this machine connected fine. I cannot say what caused the problem. This machine will connect to the internet just fine via Ethernet, and will connect via Wi-Fi if I place the machine within a foot or two from the router (which incidently is an Xfinity router). I tried running a web-speed test and got a speed of 12.66 mbps on the occasion when the Wi-Fi happened to be working momentarily. I am at a loss. Any help would be appreciated.

Wi-Fi status specs:

PHY Mode: 802.11n

BSSID: f8:ed:a5::b2:e0

CHannel: 1 (2.4 Ghz)

Security: WPA2 Personal

RsSI: -75

TRansmit rate: 65

MCS Index: 7