Hello everyone. Went through with a transmission flush and fluid change, running mercon V now, did about a half a can of seafoam the night before.

I'd been getting a weird shift, spazzing out between 2nd gear and 3rd gear, the "seems like it's in neutral" for a few seconds deal. Also I reset the ECU afterwards, (I think I did it right, battery disconnected for about an hour?). Test drives today are getting better. The car has less of a tendency to do the crazy shift. I'd say it probably happened 8 or 9 out of 10 times before, and now it's considerably lower.
I keep hearing this could be caused by a sensor, but i'm not sure which one i'm looking for, or how to clean it/change it. Anyway, I know this is a bit of a repeat topic, bu I searched around quite a lot before posting, and I just didn't feel like I could find what I was looking for.
Any input is appreciated, even if you just want to laugh and point