A few questions before I give the easy answer:
Do you use the PIX for point-to-point VPN between sites?
Do you use the PIX as a VPN concentrator for end users?
Do you have a DMZ or multiple DMZs?
Has anything recently changed in your network that would have increased traffic through your PIX?
I mention these things because they all require CPU resources and memory.
The easiest way to free up memory is to reboot the PIX. Of course, this is not always an option but the results are very predictable. If after the reboot the memory usage goes right back up to previous levels then it's time to do a traffic study of your network and determine if this PIX has the horsepower you require or if some other network re-design is in order.
Hope this helps.
answers in order:
no use point to pont vpn.
no use for VPN concentrator.
Yes, we have multiple DMZ?s (10)
Two weeks ago we upgrade the version to 7.2(2).