Load on Cisco ASA firewall after running asterisk successful


I have installed asterisk on RHEL 4 in P-IV 1.7 GHz 512 MB RAM. I am using SIP protocol. Currently I am using asterisk within our orgnisation network. Asterisk server is connected to DMZ interface of Cisco PIX firewall. 5-10 minutes after starting asterisk server cpu usage of PIX firewall shoots up to 100%. Asterisk server is putting lot of load on PIX Firewall. Why this is happening ? How can I resolve this issue ?

Thanks in advance.