Wireless dropping connection

Apologies in advance for the long post, but I'm a bit confused as to what my
wireless connection is doing!!

This is my current setup:

> Router: USR9106 - 4 port ethernet hub, wireless, and ADSL modem; 192.168.1.1
> 1x XPSP2 pc connected via ethernet; 192.168.1.3
> 1x XPSP2 pc connected in the room above via wireless Belkin NIC; 192.168.1.5
> Internet connection speed: 2mbps, PPPoA

The ethernet connection works fine, I rarely have any trouble with this at
all. I can download files at rates of around 180-200kbps regularly.

The wireless connection is not as good though. Whilst working well for the
first few months, only disconnecting about once a week, the disconnects
became more frequent until the wifi nic would slow down, drop out and not
reconnect for a few days. After replacing the NIC with a belkin one,
everything was back to square one, the odd disconnect every week which I
could live with. However, as before, I eventually started to experience
disconnects daily, even hourly and the NIC refused to reconnect. The belkin
software pops up saying there are more than one wireless networks available,
yet fails to find any when searching. Speeds when connected with the belkin
NIC are constantly changing (its a 54g card, but regularly slows right down
to 1mbps), whilst the previous NIC happily sat at 54mpbs when connected.

I've disabled the WZC service, no luck; changing channels, no luck; disabled
the belkin utility and let windows manage the connection, no luck; restarting
the pc/router/reinstalling the NIC, no luck. Enabled WEP and its worked ok
for a while, then started disconnecting regularly again. Eventually the NIC
will connect and work fine, only to drop out again a few hours later.
Currently, after another reinstall of the NIC and hard reset of the router,
the NIC works (on the belkin software) connects ok, works for about 5 hours
then drops out - still thinks its connected, but no packets are received. At
best it connects at 36mbps, usually only 24 or slows down to 18, 11, 5.5, 2.0
or 1.0 whilst not receiving packets. Currently, choosing to repair the
connection brings it back to life @ 24mbps (didn't work before the last NIC
reinstall), but surely I shouldn't have to keep doing this? Even when
connected, my top download speed from the internet seems to be about 110kbps,
more commonly about 60-70kbps (when the NIC says its connected @ 24mbps). As
far as I'm aware there's nothing new in the vicinity that could be causing
interference, the NIC is about 2.5 metres above the router, albeit through a
single floor.

Also, I've never been able to access shared areas when connected via
wireless; hooking the wireless pc up via ethernet and file sharing (and
everything else!) works fine...

Any suggestions on what I'm doing wrong? Both the router and NIC seem to
think they're Ok (router says its broadcasting, NIC says its searching but
cannot find a source), I've updated the drivers for the NIC and the firmware
on the router but to no avail. Surely even when connected at 24mbps my
download speed should be faster than 110kbps when the ethernet-connected pc
can manage double that?

Thanks,
- Jack