Runlevel5 nvidia problems

Hi all,

So i've installed the proprietary NVIDIA drivers for my P400 Quadro.
I had to drop to runlevel3 to do this, and tested them with startx and everything works fine.

When I switch back to runlevel5, my system hangs during boot-up.
Only thing i can do is pull the card, then boot with my intel card,
Dropping back to runlevel3, shutting down and reinstalling the NVIDIA, then booting and running startx, everything is working again.

So my question is, how can I find out what is causing the boot failure at runlevel5?

Kind regards,

BP