Clients on same interface not reachable after some time ...

[Beat Meier]

Hello all

I have the problem that after some time (hours or 1 day) I cannot reach 
clients
connected to the same AP.
There is no problem to reach everything else only clients on the same 
interface
are no more reachable. Restarting hostapd will solve the problems for 
some hours.
There is no message in syslog :-(
This seems to to occure at least with the Compex WLM54GP23 card.
hostapd/wpa_supplicant is 0.5.5 (madwifi-0.9.4)
Is this an hostapd issue or an madwifi issue?
Anybody the same effect with other or the same wireless card?
How can I track down the problem?

Greetings and thanks

Beat