It has been a week and I have put a few hundred miles on the car; I think everything is good. No problems have reappeared.
It seems that it was just ECU relearn. Thanks for the insight.

Marking this thread as closed.

Wow, yeah I just looked around a bit at ECU relearn. I didn't know resetting the ECU could have such pronounced effects. And come to think of it, it almost felt like the transmission was shifting somewhat smoother after this as well. I'll read up some more on the whole process; hopefully that's all ...

Yes, I did disconnect the battery.

Could the ECU still have needed to adjust anything even if the battery was disconnected?

Thanks for the reply.

TLDR: Do PCV job, car gets surging idle once warm; next day it is magically fixed. Hello MVS, first time poster looking for advice. I have lurked here before for advice, and I know you all can be a great help. This weekend I finally had a good chance to replace the PCV system on my 1998 S70; it is a...