Speed Degradation - Wireless Network

I am using a D-Link Wireless Router DI-624 for connecting two Win XP Pro PCs to
internet via wireless. I am on RCN cable connection and the said arrangement
has been working well for two years.

Lately I have started noticing that the wireless Network speed (seen by right
clicking the wireless network icon in the task bar) randomly goes down from a
normal of 11Mbps to 1 or 2Mbs or somewhere around that and it becomes impossible
to surf or check the email. Even though the wireless signal strength remains
very good to excellent through out.

Then after a variable period of time (30 minutes to several hours), the speed
returns to 11Mbps and surfing becomes normal again. During this time when the
wireless network speed goes down, one PC (3rd PC) that is wired to the router
shows normal network speed of 100Mbps and surfing on it remains possible though
moderately slower than under normal conditions.

I have ruled out any problem with the wireless adapters in the affected PCs. The
only other possibility that comes to mind is erratic router functionality. I am
not sure if any problem on the ISP side cause this too.

Any help, explanation of the phenomenon and possible trouble shooting hints
would be greatly appreciated.

Matt