Hi Sujatha,
Thanks for your reply.
I have upgraded the BIOS from the link you shown.
The link given is for VPRO machine while mine is non-VPRO.
So I upgrade the non-VPRO version of A11.
In BIOS, the screen doesn't flicking no matter if I have upgraded BIOS.
I think maybe it is the problem of HD4000 driver or BIOS's power manager interface.
I changed the Intel Graphics Settings from Balanced to maximum Performance when using battery
in advance power setting in Windows 7 balance mode.
The LCD brightness is stable all the time. No matter if I have AC connecting.
I hope DELL to fixed this issue in the furture.
Many thanks!