high idle/low idle
 

i have an '82 924 non-turbo. i'm having trouble with my idle. when i start the car and let it warm up it will idle down to 900-1000 rpm. after driving it more than ten minutes or so and shut it off, i go to restart it , the idle surges from 100 to shutting off. i can adjust the idle by-pass screw and raise the idle to where it's stable again and running fine, but when i go to start it again(normally after a day or so) the idle is higher than normal due to me adjusting the screw.is this do to an air/fuel or vacuum leak?
any help would be great.

p.s. this is a spare hobby car so it doesn't get more than maybe
400 miles a year