Standalone mode & performance

Hi, I use Qpopper 4.0.9 on Fedora Core 5 and hardware resource are:

CPU: Intel Xeon 3.00 GHz
Memory: 3 GB RAM
Disk: two hard disk from 72.8 GB/10.000 RPM/WIDE ULTRA320 SCSI in RAID 1

I have about 700 users and from a few days I have a dramatic perfomace 
problem because in some hours the queue increase in excess mode and I have 
more 800 mails in spool directory !I use qpopper in standalone mode and I 
use -s -R options:

[root@mail ~]# ps -ax|grep popper
19147 ?        S      0:00 /usr/local/sbin/popper -s -R

..if popper service is stopped the queue are emptied in a few minutes.
I have read the documentation about 'Perfomance' but I haven't solution for 
my problem, What can I do to solve my problem ?
Thanks in advance.

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   Salvatore.