The low idle screw! On the side toward the back of p7100 or follow throttle linkage. (Not to be mistaken for the high idle screw which is also on the side but further forward and is tamper proof. )
Hard to reach, 10 mm niut has to br loosen then turn screw to increase or decrease idle. When you get idle to the rpm you want tighten nut.
5 spd trans ildle 780 rpm in neutral with a/c on.
auto trans 750 to 800 rpm in drive with a/c on. (make sure parking brake is on and wheel blocked. )
The loss of idle speed is usually because the top of the set screw is wears down.