In the snap throttle video I notice that your vacuum is down to about 13 in.hg at idle.
which is low vacuum
The needle acts sort of correct on snap throttle.
But vacuum is low
I also notice your spoiler light is on.
In the next video your gauge is now (around 1:15- 1:30) in the green at around 19 in.hg
I notice a needle tick several times when holding steady RPM those are probably due to misfires.
Toward the end of the vid I see the needle tick and a light flash on the dash.
But I can't tell if it is a CEL or an oil light.
And the spoiler light is still on
You may have some restriction in you exhaust. that can cause low vacuum at idle.
Also causes needle to slowly drop at idle or steady RPM
It can cause misfires because the cylinder cannot fill with fuel/air mixture correctly.
what do your post cat O-2sensor signals look like at idle???
Can you graph them with Durametric for a few minutes and post them???
put all 4 O2 signals up on one graph if you can with RPM
