Twice today I set the idle at 1000 in park, 900 in drive, and after driving the car the idle in park shot up to 1200-1300 when I put it in park.Brought it back down with the idle screw, and when I cranked it back up it dropped down to about 500 at idle in gear...what is going on here?

The car was originally running rich and my Dad adjusted the two mixture screws a half turn clockwise, it was running good. Beat on it a little and when we got back the idle jumped up the first time. Adjusted the idle screw and went counter clockwise a quarter turn to richen it back up a little, had it idleing good,turned it off. Then when I went to leave it dropped to 500 in reverse.Brought it back up and it seemed good.

Drove 35 miles home and when I put it in park the idle shot up to 1200.

Holley 750 vacuum secondary.