Display Problem (and fix) on my Pi

I just tried my Pi on a different screen (Dell 1920×1200).
On HDMI it gave an overscannned image, in the config.txt disable_overscan=0 fixed the shape, but the display was unusably fuzzy (having been scaled twice).

On DVI it was scaled to 1024 x768 and not usable at all.

After a lot of fiddling, I discovered removing hdmi_mode=16 fixed the issue completely.

(does enabling arm_freq=16 do anything under RISC OS?)