My current F3 has a very similar problem when I first got it. My brother bought it for me and neglect to inspect it carefully enough. The meter would work fine either in manual or auto but only if the ISO setting is very high and I don't stop the lens down a lot. If the combination of lens stop down and ISO turning down from 6400 is 5 stops or greater it would simply displaying -8- in A or - in Manual. When it works the meter is correct. When I said the combination of stopping down is 5 stops I meant that if I set it at ISO 6400 it would work until I stop down past F5.6 with a f/1.4 lens. If I set it at ISO 3200 then I can only stop down to f/4 and ISO 1600 I can stop down to f/2.8, ISO 800 I can stop down to f/2 etc...
The problem was found that the subtracting amplifier that subtract the voltage from the meter coupling and ISO resistor from the voltage from photocell is malfunction. If the voltage from the meter coupling and ISO setting is greater than a certain value it simply output 0 volt. I had the shop replaced the chip and it works fine today.