Dell e6400 Maximum Memory FSB

Hi,  I have dell latitude E6400 Notebook. It had 2GB single ram installed, I installed new 1GB ram in addition to that 2GB. Thereafter screen started to freeze (some letters broken) in some places.

Also I observed that when I installed the same capacity pairs (1GB +1GB or 2GB + 2GB) it works properly.

Note. I used PC2-6400 (800 MHz) RAMs for the all combinations made to do the above tests but BIOS shows only 667 MHz instead of 800Mhz. Dell says Latitude supports Memory FSB 800 MHz.

My OS is Win7 (32x).

Please help me to overcome this issue…..

Thanks in advance.