I'll add a bit too... try to set your video card's refresh rate to the same as your powerline frequency, 50 Hz in the UK/EU and 60 Hz in North America.

The filtering in your monitor's power supply may be failing and letting the powerline ripple in to the vertical deflection circuits. By setting the refresh rate to the same, all that happens is a slight nonlinearity rather than a jitter.

Another possible source of this sort of trouble is a bad earthing connection. It's best to have your PC and monitor share a power strip rather than two different wall outlets.

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Fav quote - "Experience is whatcha don't get 'till ya don't need it no more."

System - Athlon 1.4GHz, Win98, Hauppauge PVR250 receiver and compressor.
Software -Magix Movie Edit Pro 10, Nero 6 + NeroVision Express, Moho 4.61, PSP 8.1, Bryce, Quicktime 6.52 pro, Goldwave 5, DVD-Lab.
Cameras - Panasonic GS9, Canon ES8400V, Canon EOS D20 and Canon A70