Varnish dumping cache?

[Skye Poier Nott]

Hello,

I'm a new Varnish user (coming from Squid - glad to be rid of it) but  
I'm seeing some strange behaviour when I test load a Varnish server.   
I have a configuration of 4 machines with 250 http client test threads  
each requesting a set of 46 files from 2,000 virtual hosts (92,000  
objects total).  But while it's running, I see the following:

# while ((1)); do
date
varnishstat -1 | grep n_object
sleep 5
done

n_object will climb steadily and then all of a sudden, it all goes away:

n_objecthead            86765          .   N struct objecthead
Mon Apr 14 11:51:06 PDT 2008
n_object                86780          .   N struct object
n_objecthead            86782          .   N struct objecthead
Mon Apr 14 11:51:11 PDT 2008
n_object                  358          .   N struct object
n_objecthead              358          .   N struct objecthead
Mon Apr 14 11:51:16 PDT 2008
n_object                  358          .   N struct object
n_objecthead              358          .   N struct objecthead

I don't see anything in varnishlog -i Error when this happens but I'm  
not sure where to look.
Here's my config:

param.show
200 783
user                 nobody (65534)
group                nogroup (65533)
default_ttl          86400 [seconds]
thread_pools         1 [pools]
thread_pool_max      1000 [threads]
thread_pool_min      1 [threads]
thread_pool_timeout  120 [seconds]
overflow_max         100 [%]
http_workspace       8192 [bytes]
sess_timeout         5 [seconds]
pipe_timeout         60 [seconds]
send_timeout         600 [seconds]
auto_restart         on [bool]
fetch_chunksize      128 [kilobytes]
sendfile_threshold   unlimited [bytes]
vcl_trace            off [bool]
listen_address       ":80"
listen_depth         1024 [connections]
srcaddr_hash         1049 [buckets]
srcaddr_ttl          30 [seconds]
backend_http11       off [bool]
client_http11        off [bool]
ping_interval        3 [seconds]

storage is varnishd_storage="file,/var/cache/varnish,128G" (pre-alloced)
on FreeBSD 6.3-RELEASE

Thanks for helping this Varnish newbie.  Great daemon!

Skye