The exposed center conductor could be allowing noise in I guess. Cloudy weather will reduce your snr, I see a 2.5-3 SNR drop on cloudy days. I would try adding a shield to that connection. Just use foil, being careful not to short anything, and ground the foil on an sma connector, or the shielding on the front of the pcb.
EDIT: I forgot about the sma connection its self. The center conductor is still exposed. Just cover that whole end (or the whole thing) of the LNA with foil.