This is ancient history for me and I can't even begin to remember the details on these things but there's at least a reasonable chance it's related to the distributor.
Pop the distributor cap loose and check for any sideways movement in the rotor/distributor shaft. Ideally there should be none that is noticeable. If you can feel the shaft moving noticeably side to side then the distributor bushings are worn.
Worn distributor shaft bushings was a very common problem on electronic ignition Subarus. The odd thing is that the older contact point distributors did not really suffer this problem even though the distributors were essentially the same except for the base plate inside.