High idle

When I start my car, the RPMs are really high and if I put in gear feels like I’m breaking my transmission, lol. Sometimes it happens when I’m driving and will accelerate on its own. I will shut the car on and off to see if it will go away sometimes it works most times it doesn't. I have replaced my Throttle position sensor and have an idle air control that I haven’t put in because at this point, I don't know if I need it. Please help.