running out of mbuf clusters

[John Oxley]

Hi,

I'm running a shaper system on a FreeBSD box.  It is pushing sustained 8
Mbps.  In the messages log I'm getting lots of 

Dec 11 21:03:54 ritalin /kernel: All mbuf clusters exhausted, please see
tuning(7).

When I run netstat -m however I get 
$ netstat -m
1766/5904/80000 mbufs in use (current/peak/max):
        1766 mbufs allocated to data
1765/5900/20000 mbuf clusters in use (current/peak/max)
13276 Kbytes allocated to network (22% of mb_map in use)
30 requests for memory denied
0 requests for memory delayed
0 calls to protocol drain routines


Why is it saying its run out of mbuf clusters when it peaked at 5900?
The machine is running FreeBSD 4.9-RELEASE.  The reason for this is that
it is a shaper appliance, and it was shipped to us with 4.9 on it.

Does anyone have any ideas