Sounds to me like you may have dirt in the socket or on the ram contact areas. Or, worst case scenario is the soldiering on the pins of the socket is going bad.
Check for dirt/dust, and look underneath the mobo at the pins sticking out from the socket. Make sure the solder is all there and neat, simular to the other sockets. Also look at the plastic on the socket itself, compare it to the other ones. Even a small itty bitty piece has broken off could cause problems.
If you have a reputable electronics repair shop around you could have it repaired, it could be cheap, or expensive.