Welcome!
I had a similar problem with my D5100 way back in 2013. Had to send it in for repair. I choose a Nikon authorized repair place. I had lots of problems with them.
According to the paperwork, they repaired and adjusted the main circuit and auto focus operation. They cleaned the sensor again and did general cleaning and lubrication.
Good luck!
Please let us know what the problem was in case someone else has the problem in the future.