I got a brand new Dell Latitude E5400 laptop, Windows 7, and everything's working fine, except that once I connected it to the external secondary 19' LCD screen, the LCD screen slightly flickers which is slight, but definitely noticeable. It's like a bad version of low resolution set on a monitor. The internal laptop's screen is working well. The Windows 7 32bit was pre-installed by Dell. I have 4 other identical E5400 at the office, and they all work fine with secondary screens.
I tried switching resolution on monitor from 60 to 75hz, trying a different cable and monitor, messing with the Fn key which switches display modes to external/internal, and checked for an updated driver, but there's no updates available.
What else can I try to fix or diagnose further?