If it were a “normal” problem like incompatibilities or incorrect configuration, then the erroneous results would be more consistent than what you described. Also, you said that a mem-test gave you no errors (how long/how many passes did you do), so chances are that nothing is damaged if it does indeed work. From the description you gave, I agree with billc.cn; it does sound like a contact problem which would give the intermittent (ie random) errors you are getting.
What I would recommend is to shut everything down and pull the plug (don’t just turn it off), then use a standard pencil eraser (not a hard one) to wipe off the pins on both sides of the RAM modules (and of course wipe off the rubber shavings). Next, use an old toothbrush to lightly brush the RAM slots (perhaps breathing onto them to add a slight bit of humidity).
If that does not do the trick, examine the pins on the RAM sticks. Do they look clean and shiny? If not, try repeating the cleaning. (I have in the past even resorted to using an emory board/cardboard nail file—to lightly scrub the pins on the sticks and in the slots.)