Framebuffer display "wraps around"

[Lion]

I have a Dell Latitude laptop with 1400x1050 display. The video card is Geforce4 440 Go.
Display under X is perfect. As framebuffer device I use vesafb-tng (for combination with the NVidia accelerated X-server).
When setting vesafb to a resolution of 1280x1024, picture is OK, but a bit fuzzy, because it's not the native resolution.
When I set vesafb to 1400x1050, the screen 'wraps around': the lines start 1, 2 or 3 charachters too far to the right, and the last characters of long lines wrap to the (otherwise unused) left characters of the next line.

What is going wrong here, and, more importantly, how can I fix it?