I had a similar issue, plus when I shut ours off it would give me a warning to use the E-brake as the vehicle might roll. Took it in under CPO and they ended up replacing the mechtronics unit. From what I was told it's the only serviceable part of the transmission, next step would have been a new tranny.

I'm not sure how you test one, but I'd start there since it's the only serviceable part.