Bind-9.6 and Heavy Cpu Load

[Bind]

Hello
I have SunFire V880 (2 cpu +4G Ram) and installed bind 9.6.1-P1 on solaris 
10.
but my cpu load is very high!(above 90% during the pick time)
bash-3.00# prstat -a
   PID USERNAME  SIZE   RSS STATE  PRI NICE      TIME  CPU PROCESS/NLWP      
 
   562 root     2517M 2498M cpu0     0    0 1503:30:2  95% named/5
  2394 root     3808K 3168K cpu2    59    0   0:00:00 0.9% prstat/1

here are some relatde information:
rndc status
recursive clients: 841/9900/10000
My input traffic which shown by MRTG is about "2.5" Mbps recieved 
requests(udp 53) and is normal in our network behavior during the pick time.
my question is:
does this high cpu load relate to input requests and is normall or it relate 
to something else?
also i disabled dnssec(dnssec-enable no;dnssec-validation no;) & Logging 
feature in bind,
but the cpu load didnt decrease!

anybody have idea about it?
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