SES light flashing while driving

Anyone else ever experience this?

I hadn't seen that one before.

The condition at the time of the flashing SES is a low-rpm, high load "sag" which feels from the seat of my pants like a misfire condition on one cylinder.

I believe it to have to do with plug wires needing replacement and either "cross-talking" or leaking to ground, as this just started with the rain-every-day weather we started having recently. It mitigates as the car warms up which would hint at heat driving off the moisture.

Present unresolved codes are P0128 - coolant temp sensor and P0141 - post cat O2 sensor unhappy, however the vehicle ran perfectly for several thousand miles with these stored.

I think your on the right track. Scan it out to get the specific cylinder thats missfiring. Or else do a cylinder drop test. Once you found the defective cylinder, you may want to pull the plug and see what it looks like. If wet, chances are the fuel system isnt the problem. You could also do a quick compression check just to rule out mechanical issues. From there you'll probably be all set to replace the plugs and wires.