HIGHMEM on 440EPx board with 1GB of RAM

Hi,

I ran into some trouble with a custom 440EPx board that uses 1GB of RAM 
(SO-DIMM module). The board and kernel port is very identical to the AMCC 
sequoia board.
The kernel (recent version from Denx 2.6 repository) is configured with 
HIGHMEM support.

The most obvious problems are crashes when accessing yaffs2 partitions on NAND 
flashes. This seems to be no NAND or yaffs filesystem issue.

My problems dissapear when passing the mem= kernel parameter with mem=768M. 

In general the system is stable with the full gig of ram and e.g. not using 
yaffs. So I do not believe in a hardware issue.

Can anybody give me a hint where I should start to investigate ?

Matthias