Projector on DVI makes desktop jerky (nvidia/xorg/KDE)

[JanErik]

I have an old video projector connected on a DVI port of my 8800GT (normally use a second monitor there).
But when projector is connected, it makes the video output all jerky, every second it sort of locks for a few milliseconds.

And it is the same on both monitors so it is not just the video output on the projector. Even with projector turned of it does this.

Tried connecting it with a VGA cable + converter and then it worked fine.
Tried changing refresh frequencies but it does not help. The projectors native frequency is supposed to be 43 Hz but KDE monitor settings shows noting lower than 60 Hz. Is there a way to force this?

System is up to date on ~amd64.