Comment 2 for bug 711954

[ 328.760] (II) intel(0): Modeline "1680x1050"x60.0 146.25 1680 1784 1960 2240 1050 1053 1059 1089 -hsync +vsync (65.3 kHz)

Likely a bug in the kernel's kms code, probably hardware-specific, but I'll leave against -intel and see if I can give you a hand narrowing this down a bit. What was the last version of natty that worked? Can you try booting the .37 kernel and see whether it reproduces with that?

Once you can identify a working version of the kernel, then you can git bisect down to the patch that caused the problem, and we can go from there.

My guess is that something changed in how the modeline is generated for that monitor by the kernel (or maybe monitors in general, but your monitor is hit due to being an edge case).

If you're looking for workarounds, you might try nomodeset to turn off the kernel's resolution magic, and see if X can do it better on its own.