ISC DHCP Problem

Hello,

I have a strange problem with ISC DHCP. From time to time DHCP server removes IP adresess to one cable modem and gives to another one in a short period of time. It behaves like it does not have free leases. I have checked and there are more then 3000 free leases. I dont know if it is a load problem because the leases file is very big (74 Mb)? Does ISC have a load limit?

Regards,
Dori