Problems with Apache over NFS (Or maybe any other network file system for that matter), maybe look at

http://httpd.apache.org/docs/2.2/mod/core.html#enablemmap
http://httpd.apache.org/docs/2.2/mod/core.html#enablesendfile

Long story short, disable these.
I.e.
vi /etc/apache2/conf.d/mmap.conf
add

EnableMMAP off
EnableSendfile off

HTH

Brent

*Update*
Just to add. I just came across these links.

http://www.cyberciti.biz/tips/apache-223-corrupt-file-download-issue.html
http://www.scribd.com/doc/7234985/Scaling-Apache-Handout