CGI Python user/group permission weirdness

[Aienthiwan]

Ok - this one's a baffling one.

I have a Python script run in cgi, it accesses a directory...
/var/foo.

The permissions on /var/foo are 770 with an ownership of mark:dbtest.
The permissions on /var are 755.

In my /etc/group file, www-data is part of the www-data, dbtest,
fusion and cvs groups.

I get an exception thrown with a permission denied on /var/foo.

I have confirmed that it's the www-data user by calling a
os.system('whoami') in my script for debugging.

When I su www-data at the terminal, I can access /var/foo just fine.

What's even weirder, if I switch the ownership of /var/foo to any of
the other groups (except the cvs and the dbtest one), the script
works!

The only inconsistancy is in dbtest and cvs. 

I have attempted to rename them, adjust their ids, pretty much
everything I thought of.

I'm out of ideas.

HELP!

~ Aienthiwan.