Do you keep the prob end wet all the time? If not then you should put distilled water in the prob cap. What i found with mine was while very accurate in solution. I found if i let it dry out between uses it would get freaky like that. When I'd go to calibrate i would need to let sit in the solution for 15-20min before i calibrated or it would most definitely be off. Keeping the prob end wet did away with that problem.
My solution was a small piece of cotton ball in the prob cap and keep distilled water on the prob all the time.