We have a Dell OptiPlex 755  where the diagnostic lights will often show a 3 4 pattern.  This fault usually occurs about once per week.  I realize this light pattern indicates the system has detected a memory failure, though I have replaced the two DDR2 memory modules three times over the past year.  The memory was twice replaced using new Dell modules (Nanya) and once with modules ordered from Crucial.com.

What’s the problem and how do I fix it?

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