I posted this only because many years ago a tech at the local dealer -- said to first suspect the wires/plug ends on this motor.
I ran into this last year again on the 88 (last 560 still around) young tech said he thought it was the injectors ($$) sure enough it was a plug wire or end -- I can not remember.
I wanted to post this because it can be a very strange problem -- years ago wet weather would set them off on most cars, but I have had the problem come and go and have the motor run great in wet weather and bad in dry. Only to have a plug/ wire be the cause!!
With the age at thgis point all will suffer this problem -- I have never had a problem with a rotor , cap, or coil on these cars.
In fact I have never replaced any injectors on any!