LG L204WT problem (horizontal lines appear after 10 mins)

Monitor: LG L204WT (widescreen)
Graphics Card: NVidia Geforce 6600 GT
using the DVI connection (but got the same problem when using analog con)

Bought the display a few months ago (Nov 2006) and it worked fine at first. After 1 month, shimmering horizontal lines started to appear, approximately 10 minutes after starting my computer; Usually it starts with 1 line and then they get more until I can't see a damn thing.

I called customer support @ LG, they told me Ithey would give me a new display, since it seems to be defective.

Now: it could be that they are right, and I hope they are, but I wonder if the fault could be elsewhere (gfx card etc..), and I would have the same problems again.

Any info is appreciated.

edit: switching to another resolution doesn't solve the problem.