Need help with very fast idle shifted in park

[paximus]

Hey friends and fellow chevy truck lovers, I have a question I need some help with, I am new here so first of all let me say HELLO and thanks for having such a cool group. Here is my PROBLEM: I have a 78 chevy pickup and the last week when I put it in to park (its automatic transmission) the idle surges real high and super fast and it wont go down, does anyone know what may cause that and how I go about fixing it? I am handy and have tools etc, I have just never run in to this problem, it just started happening out of nowhere, the distributor cap looks ok, I havent checked the timing but the truck was tuned up and timing set about 7 months ago and its been running fine, it runs fine now but when I shift it in to park REAL HIGH FAST IDLE like someone is flooring the gas pedal. Please help with step by step instructions, this is one of my work/fleet trucks so we need it on the road and running to make money. THANKS FOR READING AND THANKS FOR ANY HELP YOU CAN OFFER!