I also have a Dell Inspiron 5150, showing the same problems under Gentoo Linux (Windows is fine).
When attaching a projector / external monitor, the image is unreadable as the the individual lines are rapidly moving 5-10 pixels left and right, every second line in opposite directions. WORKAROUND: Booting the machine with the projector attached, and switching to the projector before X starts will work. Restarting X is not enough.
Even in this case, there is also the problem with the LCD display shifting to a ridiculously low resolution when I select the external display.
I am using Gentoo Linux with XFree 4.3.0-r3 and the 1.0.4496 nVidia driver. I have attached my XF86Config file and the X log.
PS. Is this the way to report bugs in the driver, or is there another channel for bug reports?