OK. The last things you did, did not cause the erratic idle.

My next choice would be a vacuum leak.

I would try making it drop off like you described, then when it has just done that, I would
try making it drop off again, so I would know it's "in the mood", so to speak.
I would then drive it again and when I stop, I would submerge the intake area with WD40.

Wondering about a bad gasket somewhere that when driving, air going in the intake
sucks it closed, then when stopped, the pressure to keep it closed is not there and it
loses some vacuum........and the idle drops.....

Intake manifold bolts all tight?