PIX checking MAX connections

I have a PIX 501 on a network which has just over 10 workstations, depending whos working that day. (ie turned on machines) Sometimes I get one or two workstations that can ping the gateway but not the internet until the PIX is rebooted. I assume this has to do with the base 10 user license keeping an eye on the MAC addresses. How can I check for this and is there any temp solution. ie. change the MAC address expiry timeout, etc. (just guessing here)