DLink wireless - can ping one way only reliably - suggestions?

I've never been totally happy with my wireless network but it's worked
since an afternoon playing with drivers. At least that's what I thought
because I was largely using it for ICS on one computer but when I tried
to use the network in other ways I found problems. The setup is
described below but basically the problem is that I can ping from one
system to another and get 100% success with latency of 2-3ms. Pinging
the other way (simultaneously or not) gets around 80-85% failure rate
over 50 pings. Latency is also 20 to 700 ms on those which get through.

I'm fairly happy with small wired networks but I've no idea where to
start on this (other than the comments below). Any suggestions?

The setup:

A PC running Windows 2000 using a 3COM 3C905B PCI card to connect to the
WAP via a crossover cable at 100 mbits/sec full duplex. This PC has a
USB ADSL "modem" which it shares via a proxy programme (602Pro LAN
Suite). The sharing works perfectly now that I've switched to use just
802.11b and turned off a which can't get six feet to the next room in
the house! The card has a manual IP address (say it's 192.168.0.1).
There is a firewall running normally but closing it down makes no
difference.

A PC running Windows 98 with a DLink PCI dual mode (a and b) card
(DWL520AB) which accesses the Internet (web and mail) via the proxy (IP
address 192.168.0.2). This gets 40-50% signal strength (11 mbps) which
is pathetic (over six feet) but better than the laptop gets in it's
normal location.

A laptop with a DLink wireless card (b only) IP address 192.168.0.3
running XP Pro. XP reports the signal strength as very low (good old
Microsoft what does that mean anyway 10%? 50%?) but connects at 22mbps
anyway (turbo mode but still 802.11b).

The DLink DWL6000 WAP.

More on the problem:

I have shared one folder on each system. The 98 PC can access the 2000
folder fully as can the XP laptop. The 2000 system can "see" the folder
on the 98 machine but when it tries to access it I am told it is not
accessible (path not found). I can ping (100%):

2000 to XP
XP to 2000
98 to 2000

I cannot ping reliably:

2000 to 98
XP to 98

All this suggests a problem on the 98 system but what? I've tried with
and without encryption just in case but it appears to make no
difference.

TIA