how to explain a sudden slowdown with Linksys WRT54GC?

I'm hoping that someone has some experience with the Linksys WRT54GC:

Several months ago, my next door neighbor and I agreed to set up and
share a Comcast high speed internet account/wireless network run off of
a WRT54GC. When we first set it up, the throughput was great -- I had
no problem accessing the web, and transmission speeds were what I would
consider to be excellent.

Over the last two weeks, however, we both notice a dramatic slowdown.
Web pages take an eternity to load, and often we find that we need to
keep clicking on "refresh" in our browser to make a page load. We have
called Comcast, who have reported no errors with the line.

Since our network is secure, there is no one else on the wlan, and I
have checked (using the router administration tools) to make sure that
there is no one else logged on other than the two of us (I wondered if
someone had hacked the password).

Both of us have VERY strong signal from the router, so I don't think
that this is a range issue.

The router is in my neighbors apartment, and I have asked my neighbor
to disconnect the router and plug the cable modem directly into his PC
to isolate the problem and make sure that the problem is not with the
line. I have also scheduled a time to go over there and manually reset
the router to see if that changes anything.

I am wondering if anyone has any other suggestions about what might be
wrong:

A)If this is a problem with the line, is it possible that someone is
leeching off of the connection that we have, at the level of the line
itself? (Could someone be tapping into our bandwith?) Is this a
problem with Comcast and the type of line that we have? i.e. could it
be that too many people in our building/area also use Comcast?

B)if this is a problem with the router -- is there anything that I
haven't thought of? Could it be that there is some sort of problem
with the router itself? My impression is that if there were truly
something wrong, the router wouldn't work at all.

I'd appreciate any suggestions

thank you