internal network problem

[brain salad surgery]

hi,

i have a wireless internal network that uses prism2
cards with hostapd for WPA and authentification.
a dhcp server is listening on that interface.
and iptables rules are also correctly configured

clients (they are all linux clients) connect
using the required WPA key. they can
ping and ssh each other.

after some time, they don't "see" each other anymore
(cannot ping or ssh even using their ip)
while the router can still ssh and ping each client.
Each client is still able to ssh the router and still
have access to internet and all network service.

it's not predictable, sometimes that happen 5min
after they are connected. sometimes everything
is fine during days...

deconnecting each client, restarting hostapd
and dhcpd on the router makes everything
work again upon reconnection of the clients

i had this problem since a long time,
and i've never seen any pertinent log (from
router or clients)

any hint would be appreciated