If it was the injectors going wide open like you said, it would be too rich and stall.
It's outside air getting in somehow.
I'd do the propane trick - it'd find it within seconds.
Get a cheapie propane torch, take off the nozzle - the tip, and remove the tiny orifice inside. That just leaves it as a valve.
Slip a hose over the end.
While running, gingerly spray the propane around where you think there may be a vacuum leak.
Try around the base of the throttle body, hose connections etc.
If there is air leaking in it will profoundly affect the RPM.
Then spray some along the intake gasket on the top side. Now go under it, spray along the intake manifold where it attaches to the head. I'd bet it's still leaking there.
You could use a carburetor cleaner, but most now don't burn, they are water based. Water won't make much difference, it won't do what you want.
Plus they are dangerous. When you spray it around it forms puddles. If it ever were to ignite you have a nice fire. Staring fluid has burned down more vehicles than you'd imagine.
Propane will burn in the cylinders like a low octane gasoline. It will not harm the engine or anything else.
Since propane is a gas it doesn't make puddles, and it dissipates quickly. In case it did ignite - it flashes, scares hell out of you, then it's out - no lingering fire.
That's what the professional shops use.
Keep it off the distributor!
I know "it can't be that" - try it anyway.