Great description of the problem. I think that we either have a voltage problem or a blockage somewhere. Unfortunately I think the problem may be very difficult to fix. Would like to know if you have discovered the culprit!
I have the same problem and have tried many things. It is intermittent which
1. I have changed the auto filter twice, ATF three times.
2. cleaned the selector switch which when clean solved it but the problem came back.
3. swapped out the SEM switch, it was OK
4. gave the battery a recondition charge with the Ctek and some improvement but the problem still returns.
driving me crazy as when it shifts it can be perfect and I think the problem is over but then it appears again.