Netgear WAP with large packets?

I have small home network of an ADSL router/WAP (Netgear 834GT), a
10/100 switch and a standalone WAP (Netgear WG602) wired into the
switch via a cable. An HP jetdirect wired print server connects the PCs
to a printer. Internet surfing works well from all points but printing
through the WAP is hopeless except for small (1 page) documents.
Printing via a cable to the switch works fine and wireless printing
through the ADSL router is also more reliable. We have a couple of Dell
laptops that suffer the same problems.
Thinking that there was an issue with the quantity of data I tried
pinging to see if this would reveal more info.
Ping works very reliably up to a test packet size of 20kB and then the
reliability falls off a cliff.
(all devices are running the latest firmware, clients are running with
XP)
Has anyone got anything useful to suggest?