Stange Wireless slowdown

Ok, I have a really complicated problem, so I need the experts:

I've been running a Charter cable modem (3 MB) HSI for 2 years w/o a
problem. I was using a d-link router encrypted with 64 bit wep. Well,
recently, everything started horribly slowing down. My wireless speeds
went from 3mb/s to under 250k. I directly connected the modem to my
computer, and it showed 3MB download. So, I assumed the router was the
problem. I switched out the D-link for a linksys, but got the exact
same problem--wireless connections are ridiculously slow. I tried
switching the SSID channel, but it didn't do anything different. I
can't figure out for the life of my what the problem is. It seems like
the modem is fine. And it's unlikely two different routers have the
same problem. Any ideas?

Is it possible that there is intereference from other devices/networks
nearby?

I once had a client who's wireless was ridiculously flaky.. it would
drop connection constantly, be very slow when it did work, and the
range was terrible. It turned out that his neighbor was using a wifi
device on the same channel, but had turned off the ssid broadcast. A
quick spectrum analysis showed a load of traffic on his channel, and
after discovering that, we were back in business.

If you have the same issue with two routers, is it possible that the
connection (cable) between the router and the modem is at fault? Did
you reboot the modem after each reconnect?

Dlinks love to use factory settings of Channel 6; try using channel
1-3, or 8-11 and see if there's any difference in performance. I'm
still suspecting interference from another wifi source.

-Luke