LOTS of lost packets

Hello,

the problem of lost packets persists. I've uselessly
upgraded the AP (wireless router/gateway/switch/etc.)
firmware with the latest release... no changes.

However I've better studied the problem.

Recall the configuration:

---- first floor -----------
PC A and PC C 
both Roper Freelan PCI Prism54 card

---- ground level ------------
wireless AP / router / etc. 
US Robotics 9106 

---- underground ----------
PC B 
SMC 2802W PCI Prism54 card

--------------------------------

Using "ping -f" as a measurement tool, I can say that
communication between each PC and the AP gives only
about 2% - 5% packet loss.

However, communication between two PCs (in any combination)
results in 60% - 70% packet loss, which is in practice
an unusable network, I can't even do a ssh session
(not to mention a file transfer).

This also includes accessing PCs external to my LAN.

I've found that, if I let a "ping -f router" endlessly run
on each of the PCs, then the loss of packets in PC-PC
communication drops from 60%-70% to just about 10%-20%,
which makes it possible to do ssh sessions and even transfer
small files at about 40-50 KB/s withoug hanging.

Do anybody have a hint for making my network function
without the need of running a ping -f in background,
and getting a more reasonable bandwidth ?

Thanks,
Franco