Vacuum reading help.

I've been developed an erratic idling problem over the past few weeks, I'm hoping for some insight as I do not think it is a vacuum leak. I'll try to provide as much info as I can...

This is a stock /6 with Carter 1BBL.

I first assumed it was a leak, I tried multiple times to find a leak by spraying WD-40 all around the intake where it meets the head, carb base, throttle body, throttle shaft - no change in RPM.

Read some other threads about vacuum leak symptoms that were actually related to a dirty carb, so I pulled it apart, cleaned it thoroughly, new gaskets etc; issue still remains.

Put a vacuum gauge on the carb at the choke pull off inlet (with engine warm) and reads a shaky 20-21in but can sometimes bump down to 18 or even 15in. I took a video of the bouncy needle but can't get it to upload here.

For in town driving, the issue is bad enough to stall frequently at red lights..
What are some opinions?