I just picked up a FG-20 in really nice cosmetic condition and what I though was perfect working condition. The problem I see now is that the meter is erratic in aperture priority. Sometimes it will give me the correct exposure and most of the time it's 1/90th or so and the meter needle will jump around. The batteries are fine and the meter works great in manual and doesn't jump around. All of the shutter speeds are fine in manual also. Any ideas?