> [ added RAM, kernel and user processes now die ]
 >
 > ... The RAM modules were tested in another machine, before we
 > added them, so I suppose they are okay.) ...
 >

Bzzt. Sorry, its not so easy. The other system board might have been
more tolerant of a timing or organizational difference, it might have
had less (or more) load on its ram bus (thus producing a different
pattern of address and data reflections), or any one of a dozen other
things.

You probably weren't grounded when swapping modules, so an ESD-related
failure might have occurred while handling them.

You can try a pencil eraser, gently, on gold contacts. Tin-lead
contacts should just be cycled a few times. If the sockets have one
plating and the modules another, check for fretting, tin-lead deposits
on the gold and try cleaning up the gold with the eraser. Blow off contacts
with an air can.

Try swapping in the new modules and extracting the present ones, one
SIMM at a time, if they are compatible. Boot up at each increment.

It's almost certainly the hardware. Also check bios setup for RAM
timing and other enables...