LOTS of packet loss

Hello, I had partial success in setting up my prism54-based
wireless network (tanks again to all persons in this list who 
helped) but the performances are so poor that the network
is in practice unusable.

The configuration is:   PC A  <--> wireless AP <-->  PC B

If PC A does a "ping -f" to AP it gets a 2%-3% packet loss
If PC B does a "ping -f" to AP it gets a 4%-5% packet loss
But if PC A pings PC B  the lost packets are 60%-70% or more.

In practice, if I try to connect with ssh from A to B
I can't even type commands by hand (not to mention transfer
files or do a NFS mount...)

I wonder it may be a "hidden node" problem
but RTS seems to be enabled.

How could I correct the problem?

Thanks,
	Franco

P.S. I have already tried every channel from 1 to 13,
some work better, some worse, but the problem is always present