1992 5.7l hot idle problem

Problem began recently, 175,000 miles. Starts great, cold engine runs fine......... warm engine in park idle erratic jumping from approx 900 rpm to almost zero ....... "catches" itself at low rpm and speeds back up to 900 rpm. Repeats over and over agin ........ if in gear it will do same thing but stall out. Seems to run fine while driving at every speed.

Thanks for any insight.