"I finally came to the conclusion that the M/B does not match 'Frequency of Memory sticks'. "
Sorry but that makes no sense to me.
Have you changed any RAM? - Added any RAM?
Motherboards will run certain specifications of RAM. You shouldn't mix specifications - if you do it is possible you will get intermittent errors - this depends a lot on each motherboard design.
Sounds quite like you may have some faulty RAM - a system giving these kinds of problems passed through my hands recently - chasnging the RAM cured it.