MAC address issue

We have approximately 65 GC108P switches on a campus network. They are locally managed (we are not using Insight) and are currently on firmware version 1.0.6.3 (but this issue happend on ver. 1.0.5.6 as well). All the ports are untagged with the default VLAN 1, and some are additionally tagged with VLAN 10 (for VOIP phones). Occasionally a computer connected to one of the ports looses connectivity, and when this happens the MAC address of the computer cannot be found in the Address Table of the switch. So although the computer has a valid IP address previously received from the main router DHCP, it cannot get past the GC108P switch or go anywhere. When this happens, I have tried to assign a static IP to the computer in the same subnet, but since the MAC address has not populated the GC108P Address Table it doesn't make a difference, its still dropped by the switch. The only solution that works is to reboot the switch, then the computer regains connectivity and its MAC address can be found in the Address Table again. I have disabled MAC Address Validation in the GC108P, but the problem continues. Today for some reason it happend to 4 computers on the LAN, on 4 different GC108P's. Thank you for any help with this issue.