We have this problem with a 535 pix.
Cisco sais that the pix can handle 1.7GBit of traffic.
With a tac case lasting for nearly a month, five different TAC engineers, one sales and dev included, we found out that with version 7.0 (and 7.1), TCP traffic and multiple context's (we use three), pix is only able to handle 300-450MBit's of traffic. The cpu is bordering traffic. 7.0 and above has much more features, and though can handle not so much traffic.
7.0.4(8) (interim release) is a optimized version, but the difference in cpu utilization is not very big. You see the difference in show processes, the processes "557poll" and "snp_timer_thread" are not there.
How much traffic must you handle?
Feed a TAC engineer with the following output:
show cpu usage context all
show cpu usage context admin
show cp usage
show process
show log
show xlate count
show conn count
show local-host | include host | count/limit
show traffic
show tech
capture abc type asp-drop all
wait for 1 minute
no capture abc type asp-drop all
show capture abc
best regards
Bernhard
