I've not handled a Spotmatic F but I do have an original Spotty.

I always thought that when using the pre-SMC Takumars on the F (or ES or ES II) the stop-down switch worked just like that on the original - set lens to Auto, then when you push the switch it'll stop the lens down and turn the meter on?

I would suspect the film speed selector if your metering is wobbly, both my KM and K1000 (essentially the same mechanism) have problems there.