Well if you tested the CPU in a different board and all the ram slots work, probably should replace the board, see if its still under warranty.
If not, try a bios update, if its up to date, try and older bios and see if it changes anything, If not, well, gonna have to deal with it or replace it.
I would also try a different CPU if you can in that board and see if it acts any different with them slots.
Last thing that could maybe fix it if all else fails, take the board out, disconnect everything, and get some contact cleaner, and preferably a piece of card board about the same thickness of the the stick of ram and insert it like a stick of ram in the bad slots while the contact cleaner is still wet, should clean the contacts up in the slot.
Though thats a last ditch effort, I wouldn't do this unless everything else fails.
Good Luck!