Netgear MR814 problems, need help

I have Comcast high-speed Internet, connecting to a Netgear MR814 v2 wireles router. The router is set as a DHCP server, with a MTU of 1500 bytes and 64-bit encryption and MAC filtering on the wireless connection. The past month or so, I've been having big problems with my Internet connection. The connection intermittently drops, and it often takes 4 or 5 attempts for a page to load. I'll often get Server Not Found. This is sort of cyclical, sometimes things will load on the first attempt, but it still takes 30+ seconds. What's more bizarre is that this also happens when I connect through the router with a cable (disabling the wireless adapter on my machine). When I connect directly to the cable modem, there are no speed issues, so this does not appear to be an issue with Comcast.

I'm usually running one or two machines on the network, either two with WinXP Pro, or one WinXP and one OS X. There are no bittorrent or similar services running, it's basic web browsing, sometimes AIM, and occasionally my VPN for work. Even with no VPN or AIM running, the problems are the same. The router has the latest firmware, and there is no port forwarding or filtering.

The slow connection happens on any computer connected to the network, so I don't think it's spyware or anything like that. Something has to be up with the router, I think. Does anyone have any suggestions?