+1

I'm with CNY-Bikeman in saying that there is a total failure to use logic in what's been done so far. The common denominator in the issue is the middle ring which is the only place the problem manifests consistently, and where it manifests regardless of all other factors.

Logic would dictate that the first thing replaced would be the ring itself.

But be forewarned that that may not resolve the problem entirely. As mentioned there is chordal action which you may feel because of the way you use the ring (higher pedal load compared to when on the outer ring).

It may also be a question of the ring's surface and tooth cut which will resolve with wear and your choice or lack of chain lube which makes it more sensitive to less than perfectly smooth engagement. I'd wait and give it time to resolve through wear, and/or (consider the source here) try a heavier, more lubricious chain oil.