@rek i had the same problem on a non-Pro 2010 Macbook (unibody white) [or maybe it was on my 2012 MBP, not sure anymore], one of the two ram slots was faulty too.
A few times when i opened the laptop, removed & put back the ram, it worked again for a while (or the opposite when it was working before i opened it >__>) ...
IIRC i ended up using only 1 ram slot too similarly, with one 8gb module