Transmission randomly pops back into first gear?

When I was driving, my car would randomly go into first gear and stay there. My RPMs would shoot up high (above 4k) at low speeds and when I turn my car off and back on, it would go back to normal, but I will still feel jerking when I brake and come to a stop. I took it in and got my mechatronics box replaced. But I still have the same problem. The only thing different is that now I have additional problems, such as the feel/noise of metal grinding when I start accelerating and car not sounding right. Also, there was one time where I turned my car off then turned back on and my temp gauge went a little higher than operating temperature. But then I turned it off and on again and the temperature went back to normal operating temperature. Again, still hearing grinding noises and shifting gears are smooth, but will randomly pop back into first gear.