Lighttpd

I've got a bunch of servers running lighttpd 1.4.20 and I'm trying to maximise throughput on the network interface. On one class of server, this is not an issue. I can push it up to 100Mb/s without an issue. On the other class of server, which has the same configuration, I can't get past 80Mb/s (95percentile). It will push up to and slightly over 80Mb/s for a short period of time, then drop significantly below (30Mb/s), as if in response to being pushed too far. Here are the particulars:

OS: FC6
Hardware (working class of servers): Intel P4 3 GHz, 1GB RAM
Hardware (problem class of servers): Dual quad-core Intel Xeon 1.6 GHz, 3GB RAM (a later purchase, hence the higher spec.)
lighttpd version: 1.4.20 (started on 1.4.18 and upgraded in order to attempt to resolve, but no luck).

Servers are serving a mix of large video files and smaller thumbnails. The majority of the requests are for thumbnails (probably a 40:1 ratio).

Any help would be greatly appreciated. I've got a total 80Mb/s I can't use because of this, which ain't cheap.

Output of vmstat on a working server:

procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu------
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 0  1     60  25160  14092 590648    0    0    87    34    1    2  9 19 45 28   0
 0  1     60  33132  14228 585256    0    0 10171   882 10642  229  1 25 10 64  0
 0  1     60  11032  13928 608972    0    0 10971    94 9432  170  1 22  3 74   0
 0  1     60  12304  13268 606416    0    0 10831     2 10516  152  1 24  2 73  0

Output of vmstat on a problem server:

procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu------
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa st
 0  3     68 125468  37164 2972584    0    0    15     3    0    0  5  1 87  8  0
 1  1     68 126848  36116 2969212    0    0  8802   317 9784  212  0  1 87 12  0
 0  3     68 125456  35856 2978272    0    0  5914  1379 6716  201  0  0 87 12  0
 0  3     68 125060  35732 2978272    0    0  9055   279 8295  217  0  1 87 12  0

Config follows (paths, mod_secdownload specifics and mimetypes left out for the sake of security and brevity):

server.modules = ( "mod_access", "mod_status", "mod_setenv", "mod_secdownload", "mod_accesslog" )

server.max-keep-alive-requests = 4
server.max-keep-alive-idle = 4
server.event-handler = "linux-sysepoll"
server.max-fds = 2048
server.stat-cache-engine = "fam"
server.follow-symlink = "enable"
server.network-backend = "writev"

Thanks for your help.