Lighttpd

I have a php script which serves up thumbnail images, using lighty 1.4.28

This script previously used fpassthru to serve the image files, but I recently switched to using X-LIGHTTPD-send-file.

Since then, I occasionally see jpgs where the image ends prematurely, and the rest is grey pixel area. Im having a hard time understanding what causes this, its like the response is ending prematurely. Forcing a refresh of the images in the browser, usually loads the image fully.
This effect has been seen in latest Firefox and Chrome.

Checking lighty access log for an image which failed, the log does suggest the full image data is being sent (ie in the response size, in bytes matches the file size)
The server is Centos 5.5 (32-bit), and is not under any particularly heavy load.

Lighty is serving up an average of 80 requests/sec on this server, the majority are via the image php script, but there are also some static jpgs and gifs which are not served via a php script.

Any suggestions welcome, thanks

Code to serve images (the script is fairly lightweight, with just some path manipulation and no database hit). I set $etag to be just the image filename, since the images never change

header("Content-type: image/jpeg");
header("ETag: \"" . $etag . "\"");
header("Last-Modified: Thu, 01 Jan 2004 00:00:00 GMT");
header("X-LIGHTTPD-send-file: " . realpath($filename));

Relevant bits of lighty config....

server.max-keep-alive-requests = 50
server.max-keep-alive-idle = 5
server.max-read-idle = 15
server.max-write-idle = 15
server.max-fds = 2048

fastcgi.server = ( ".php" => (( 
        "allow-x-send-file" => "enable",
                     "bin-path" => "/usr/local/bin/php-cgi",
                     "socket" => "/tmp/php-fastcgi.socket" + var.PID,
                     "max-procs" => 1,
                     "bin-environment" => (
                        "PHP_FCGI_CHILDREN" => "256",
                        "PHP_FCGI_MAX_REQUESTS" => "10000" 
            ),
                     "bin-copy-environment" => (
                       "PATH", "SHELL", "USER" 
                     ),
                     "broken-scriptfilename" => "enable" 
                 )))

server.max-worker = 1
etag.use-inode = "enable" 
etag.use-mtime = "enable" 
etag.use-size = "enable" 
static-file.etags = "enable"