[ltp] Memory problems on my AP21

[Constant Brouerius van Nidek]

After one of the two 128MB memory cards of my AP21 broke down, I found a new 
Kingston 256MB module (KTM-TP390X/256) which according to Kingston is the 
memory module for my machine.

What I found out is that this module does not want to sit in the memory 
compartment with its smaller Toshiba module. Depending on which I put right 
or left it shows 64MB memory, 256MB or 128MB. Running the latest memory test 
from Linux which shows the right amount of memory,  I end up with thousands 
of errors within 5 minutes if both cards are found together.  Reason why I 
have stopped that exercise.
If I run Memtest86 on each single card for an extensive period no errors are 
showing up. Is it a known fact that mem cards of different sizes do not mix 
or is there something else?