IP CONFLICT

Hi, tech people.
I manage a Wireless ISP.
I have clients with outdoor wireless unit and his PC wired to it.
The PCs have an 192.168.x.2 IP and the wireless unit a 192.168.x.254 IP.
The x is diferent for each client, of course.
I have a Redhat 9 linux system doing NAT and PROXY between my clients networks and internet.
Some days ago, suddenly, PCs started to give an IP conflict error:
“The system has detected an IP address conflict with another system on the network”
The behavior is randomic. There is no fixed time or hour to happen. Neither fixed PC. Afected PC solves after 5 or 6 restarts.
Excuse my poor english. And help me with this crazy problem, please.