RPMs shoot up?

Truck is manual, when I was driving and came to a stop the rpm would stay up when I took it out of whatever gear it was in and if I put it back in gear RPMs would go back down (A/C would be on) if I turned A/C off it would stop doing it but now it’s gotten worse. At first it would stay at a steady high rpm now when I come to a stop the RPMs keep climbing unless I turn truck off but if I turn it back on its back to normal until I take off again and come back to a stop take it out of whatever gear it’s in and RPMs steadily start climbing unless I turn it off again (and turning A/C off doesn’t matter anymore it still does it). would really appreciate your help.