I've seen Q-Jets where the throttle blades don't return to proper position because they're hanging-up on a gasket.
Worn throttle shaft bushings are known for screwing with idle speed problems, too.
When my Holley tri-power had erratic idle speed, the throttle blade screws were loose. I got lucky--they didn't actually come out, just loose enough that the blades moved around on the shaft.
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