[exim] Strange behaviour and delays

Hi,

I am currently using exim 4.30 on a sunfire v440.
Currently have smtp_accept_max = 70
I have noticed that when the number of established connections hits
the 70 mark, there is a large delay between the client sending the
terminating dot in the smtp conversations and exim giving the 250 ok,
this delay can typically be from 12 secs to 30secs, which is an awful
long time.
The cpu's =are about 45% idle, load average about 1.0 and absolutely
no disk I/O bottlenecks...

Has anyone else experienced this?
Are there any recommended kernel parameters on solaris 9 to tune for exim?

many thanks

Steve