Time to report my findings... I am not certain but this may have been the problem...

I did find the derailleur pinch bolt not as tight as if I would have tightened it (I'm gonna buy a torque wrench soon - still hard to believe that my LBS missed it not once but twice), but it was snug and obviously holding the cable well enough to control shifting 98% of the time. The problem was that the other 2% of the time it was very frustrating. I guess eventually, it would have kept getting worse and worse. After I had time to work on the bike, I still had to wait for the cold rain stop falling before I could test my adjustments. One ride with no problems, so I hope that was the solution. If it acts up again, I'll be back. Thanks, Al1943.