Hi and thanks for using 2carpros.com. Honestly, it could be many things; however, I am a bit concerned when you say there are black spots in the fluid. What I would suggest is to try a simple service. If the filter is blocked or partially blocked, it can cause this type of a problem.
To start you off, here is a link to help possible problems:
https://www.2carpros.com/articles/automatic-transmission-problems
Here are general directions to service a transmission:
https://www.2carpros.com/articles/how-to-service-an-automatic-transmission
Here is a video of a transmission being serviced:
https://youtu.be/FglBsUEOSEk
If you decide to service the transmission, make sure that it is clean and dry when you replace the pan and gasket. I have attached a picture specific to your vehicle which identifies the torque sequence. Tighten the bolts and studs in the indicated sequence to 13 N-m (1.3 kgf-m, 10 ft-lbf).
I hope this helps and you are able to resolve the problem.
Cheers,
Joe
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Monday, May 7th, 2018 AT 7:32 PM