T400s-intermittent blurriness in screen

[nanotimer]

Hi,
I own T400s. Since last few days, I've been seeing intermittent blurriness in the LED screen. The same problem is coming in externally connected LCD monitor, so I guess it is not a screen problem. I re-formatted the thinkpad and updated the drivers and still have the same problem. Lenovo hardware test shows everything passed. Any idea what can be a problem?

Many thanks,