highmem problem with lc2000

[Stefano Carlotto <stefano.carlotto@ampersand.it>]

I'm having this strange problem: hp lc2000 (sym53c896 scsi), 2 pentium 
III, kernel 2.4.22, just upgraded to 1664MB, raid software used, 
slackware 8.1.
Obviously, if i do not activate highmem support on kernel, I can see 
only 896MB, as I had before the memory upgrade.
If I activate high mem support (4GB), the system starts normally, until 
it goes multiuser: then it slows in a incredible way, almost stop, no 
disk activity, no ping answer, if I press return on keyboard, it scrolls 
very very slooooow... ( vesa framebuffer used)
any idea? :(

this is /proc/mtrr  ( no highmem support compiled)
reg00: base=0x00000000 (   0MB), size=1024MB: write-back, count=1
reg01: base=0x40000000 (1024MB), size= 512MB: write-back, count=1
reg02: base=0x60000000 (1536MB), size= 128MB: write-back, count=1


thanks
Stefano