Odd screen shimmer when using discrete graphics on T400.

[avzay]

My laptop just came back from screen replacement (due to discoloration) and in the last couple of days i noticed a weird artifact that happens only when it runs wiht discrete graphics. When certain shades of color are displayed, in that areas I can notice fine diagonal lines running through (it becomes very pronounced when I tilt the screen away from me). It is rather annoying and looks similar to this, mostly on areas with subtle change in colors:
http://www.notebookreview.com/assets/10242.jpg

When I switch to integrated graphics, the artifact goes away.

What can be going wrong here? The laptop has a WXGA+ LED panel.