I have just deployed a freshly-imaged Lenovo T420s laptop (win7x64) to a user with dual external monitors. The laptop sits in a docking station and the lid to the laptop remains closed - only external monitors are used when docked. Immediately after setting up the user's two external monitors, I noticed something odd - the main monitor is fine, but the secondary, extended monitor has flashing purple horizontal lines from top to bottom.
1. When I switch the main monitor in Windows to be the secondary, extended monitor the lines follow the monitor - the problem happens on whatever monitor is designated as the extended monitor.
2. When I put my own laptop in the user's docking station, the problem disappears....confirming the issue is with the user's laptop and not the monitors, cables, power, etc.
3. Updating the graphics card driver (Intel HD Graphics Driver 3000) on user's laptop from Lenovo's web site did nothing - the horizontal lines remain.
4. Changing resolution settings on extended monitor did nothing - horizontal lines remain.
5. I compared the display and driver settings on the user's laptop to my own laptop and the settings are identical.
I'm normally pretty darn good at diagnosing these types of problems, but this one has me stumped....any suggestions are appreciated!