Memory vanishing

So I've been having this recurring problem with my Inspiron 5100 notebook for the last year or so.  I have 2-128 Mb memory cards installed in my laptop for a total of 256 Mb.  Whenever I go to restart my computer, for some reason, the computer fails to recognize that both cards are there and I am then only running on 128 Mb.  The only way for me to get back to 256 Mb is to shut off my laptop, open the memory slot, remove the cards, and replace them.  This wouldn't be such a big deal, but one time, just by moving my laptop, the same problem occurred while running windows...I got the "Blue Screen of Death" and my computer rebooted running with 128 Mb.  I use this notebook for giving presentations and such, so I cannot have this happening at conferences and meetings.  I have sent my notebook to Dell 2x now for this issue; the first time the memory was replaced and the BIOS was updated, and the second time, those two problems were again addressed, and the CPU was replaced along with the heat sink.  I just cannot seem to solve this problem.  Is this just something I'm going to have to live with or can this be fixed???