Re: Engine or Trans Issue, Need your opinion
Short answer based on what your describing, I'd lean towards the transmission.
If it idles okay, and rev's smoothly while out of gear (Neutral), that wouldn't completely remove an engine issue, but it would be a good sign.
I know the Service interval on the Manual is stated at 100k miles, but I believe the Auto comes in under that, maybe it's 50k miles.
On my Auto cars in the past, I would typically have a Tranny Service done at least every 40k or 3 years, whichever came first.
It might be as simple as cleaning the screen, and refreshing the Tranny fluid...