I have found MemTest very good and it is often recommended in the PCA Helpline section of the magazine.
I would suggest that as your PC is under warranty you take it back and tell them what MemTest revealed. They can also run the test while you are there to prove your findings.
There is a possibility that a module needs re-seating (bad contacts) but equally memory modules do fail. It may also be worthwhile finding out who manufactured the units in your PC as there are `cheap` items that sometimes don`t work very well.