Ok, this is a problem with certain revisions of the A1200 motherboard, and isn't anything to do with the ID4 timing problems.
From the
Eyetech website:
Changes to the A1200's video circuitry to meet new EC interference emission standards means that some monitors exhibit horizontal 'tearing' of the screen image when using the PC-compatible screen modes of DBLPAL, DBLNTSC, Super72, Productivity etc. (Standard 15 kHz screen modes work fine).
Eyetech also do a fix for it. Check the link.
