random connection problems with WPA PSK encryptie (senao AP)

I have a strange problem with WPA PSK

I have a school with 2 Senao 3054CB3 de luxe AP's and 32 notebooks with
embedded Intel 2200 BG wireless card.

We installed WPA PSK and everything worked perfectly. They tested with
all kinds of datatransfer
(DHCP and domain + internet, download of patches etc. all 100% ok)

After a few days one or two notebooks could log in the domain. The next
day one or more other machines couldn't log in the network. Restarting
the PC sometimes helped connecting to the network.

After some days more PC's seemed to have the same problem. It occured
fully random to the clients.

We changed the Windows zero configuration to the latestIntel Pro set
software for the 2200 BG wireless card. No difference measurable !!

Several times we tested with the one and other accespoint to exclude an
defective accespoint. I the wireless troubleshooterlog we read that the
authentication failed (due to WPA encryption)

At last we reduced the encryption to WEP instead of WPA and everything
works nice (just like the first day's with encryption) Even 32 notebooks
on 1 AP al starting together and asking for a userprofile on the network
works stable

It looks like that 16 notebooks on an (Senao) AP with WPA is not stable
!! I heard a supplier declaring that more than 6 clients with WPA to an
AP is always a hassle ????

I never heard or read any remarks that the CPU of an AP could not handle
this amount of clients.

Unfortunately I was not able to exchange the AP for another type or brand.

(Found also out that Dlink AP's also have some trouble with WPA PSK, I
bought 2 identical units and configured them both identically. One
worked fine one didn't with WPA with Intel Centrino embedded card, WQEP
worked fine on both)

Has anyone any experience with 16 or more notebooks (Intel Centrino) on
1 accespoint and WPA PSK configured.

Is this an Senao problem or is this a common problem of WPA PSK ?

Regards in advance,

Gert