The throttle blade stop screw got adjusted.

Right above the red circle on the right hand side is the idle screw. Turn it counter clockwise to increase idle speed.

Mine was running rich when the O2 sesnor went bad. It took a while before the check engine light came on. The coolant temperature sensor on the front of the intake could be malfunctioning too. You can check it with a meter. Leaking fuel injector and cold start injector could cause it to run rich.

Anyway, rich is not good because it causes fuel wash in the cylinders which can lead to accelerated wear.

 

Release lock nut, back out screw untill its not touching, drive screw in untill it \"just touches" then add a quarter turn. Secure lock nut, verify vacuum port to throttle plate position.

So first you want to restore what the shop did. While you're there you may want to remove the dash pot, atleast make sure it has its hose to the filter, has a filter and they both pass air. (These are notorious for causing eratic idle, they can be replaced I think with new ones still but most people report they removed it instead.. Very likely a root issue with your idle)

Next you need to check/reset the timing and idle speed. You need a tachometer, a socket (10-12mm), timing light, flat blade screwdriver, and your SST for diagnostics mode (paper clip)..

Would be a good idea to have a new oring for the idle screw on hand, it's another leading cause of eratic idle.

As mentioned the idle set screw is in your photo, it's about 1cm and runs vertically along the right side of the throttle body. If it wiggles in the housing, rotates by its self, or doesn't have an intact plump oring replace the oring.

While you are in the area of the throttle body clean all the exposed bits of the rotating assembly (springs), it should be clean and dry (we are back to rocks/debris look at that!). The throttle plate shaft should be firm, no side to side up and down movement. The throttle plate should always return to the same position when it snaps closed a worn throttle body, a bad dash pot, or debris will prevent this.