It works on some installations and then it completely falls over on others.
Yes, it's the usual
Fatal error: Class 'Mage_Install_Controller_Router_Install' not found
Warning: include(Mage/Install/Controller/Router/Install.php) [function.include]:failed to open stream: No such file or directory
No compiler is not and has not been enabled
Yes the files exists
Yes rm -fr cache
Yes the permissions are correct
No there is no other server cache that needs updating
Yes disabled all other modules
Yes checked local/Mage directory
Any other suggestions?
Reverted back to the previous config.xml files for the moment whilst I put the kettle on.
Do you have any server-side caching technologies like PHP APC running?
It is possible your patch has installed correctly but APC is caching the old pages/functions and thus causing an error.
You can normally purge any such cache by restarting Apache or PHP-FPM or worst case the entire server.
Not an answer to your problem but your insight will help on a related issue.
Does the website run ok with app/code/core/Mage/Install/etc/config.xml reverted back to the original with the rest of SUPEE-5994 installed?
I'm asking because some people are having issues with 3rd party modules causing 404 status errors in the admin backend due to the fact that said modules weren't properly programmed to have proper security.
The correct answer is "Contact your module developer for a patch so we can run the patch", but expediency says "do what you can to keep the website up until you get the patch for the patch". Alas, what a cruel world, sigh, sigh, groan!