dhcpd high memory usage

It seems my dhcpd has a high memory usage. few days ago the global memory usage was about 80%, in " diag sys top" dhcpd memory column was at something like 50. I had to " dhcp lease-clear all" to fix it The memory usage is growing again. and at this time dhcps is at 25 in the memory column of " diag sys top" I have 5 dhcp servers configured (one for each vlan). lease time is 3 hours. ??? Dunno where I have to look for. Fortigate 200B Firmware Version v5.0,build0147 (GA Patch 1)