Hey dark.
Since these were working at one point, I think it's safe to assume that this would be a hardware problem. Unless your hiding some qualifications and skills, the term would be to replace the faulty component or board rather than fix.
What make and model motherboard and
RAM do you have? and also, have you performed any hardware changes around the time this problem occured? Have you tried using the
RAM from the other two slots to make sure it's a slot problem as apposed to a
RAM issue?