Check the cables and supply voltage according to the instructions here: http://elinux.org/Rpi_Camera_Module#Troubleshooting
Power limits seem to be the most common source of "it just doesn't work" issues. The RasPi is officially spec'd to draw 720mA at maximum load, and uses about 550mA in normal operation. The camera wants another 260mA, taking the total current consumption up to about 800mA.
Unfortunately, the F3 polyfuse that protects the RasPi from spikes on the supply rails is rated for a trip current of 750-800mA. The worst combination of partial factors doesn't shut down the RasPi, it just causes the voltage on the supply rails to droop a little. The droop is enough to make the system's performance unstable, without giving you a full-scale "having trouble with power, shutting down now" warning.
When you void a product warranty, you give up your right to sue the manufacturer if something goes wrong and accept full responsibility for whatever happens next. And then you truly own the product.