I do not have your hardware software set up so could not tell you if this is also true for other system of the same "type/setup", and you may already have coved most of what I would look for but I would double check the following thinks on each system, try and narrow down your search and only record fact,before you jump to conclusions
at around crash time what do files in /var/log/* log on each system, make a compare list
Is there a pattern 'only on week days' never during week end ?... only on week end... never at night etc ..
also I would not limit myself to the system, (hardware and software) but would have a look at all surrounding environment, like perhaps the are there any other electrical appliance that would or could only run every 1,2 months and cause theses problems.
The above reply is vague, but we realy need more narrowed down informations before whe can get more specific...,
Keep at it, and report here, bets are we will be able to help.
Enjoy life.
Jean-Pierre Huc
Smile I will feel the difference