Need Some Help... Transmission Issues

Hi all! I have owned a 2014 4Runner Limited with 92k miles for the past year. Everything has been great with it until the last few weeks where I've been having some transmission issues and I was wondering If anyone had any advice or have seen this issue before. In my area it is nothing but hills/mountains so if I pull out of my driveway I'm already going down hill. The past few weeks the second I put it into drive and take my foot off the gas it instantly rolls and then when I step on the gas it's like I'm revving in neutral. After a few seconds of revving it will suddenly SLAM very aggressively into first gear then it will drive perfectly normal without any issue - it's just first gear going into drive on inclines. Even if it is the slightest incline from P->D it will show this behavior. I had it towed to the transmission shop thinking it was a transmission issue but the shop mechanic is stumped trying to figure out what's wrong. The transmission operates perfectly after this and 1-5 gears are smooth as butter. Anyone have this before or hear of this happening?

 

Have you ever had the transmission fluid changed?

 

@Slopemaster not yet, I'm planning on getting it done at this shop but not sure if its the best idea in case they need to do a complete rebuild on it