a PF Prediction Error (6BG) on sodium

Ortho Vitros 350 We keep getting this PF Prediction error on Chloride and sodium. Sometime it will say test dropped, with no results. I have replace the electrometer (used) and it worked for 2 weeks and now happening again. I cleaned the plastic blade and that helped for a day. Before I replace the electrometer, we could not calibrate any of the electrolytes. when I put a different electrometer on, it worked for two weeks. Ortho tells the lab techs it is an atmosphere problem, she does not think so. Does anyone have an idea what to do next before we can't run these test again? Thank you. Rick