Faulty throttle cable?
 

I have been having this issue on and off. It's hard to replicate and I can go 3-4 days without noticing and then on the 5th day it will happen briefly and then won't notice it for another 4-5 days.

-RPMs will jump when you rev the car while stopped in neutral
-RPMs will hold for 1 second
-If you are in gear and you lift of the gas pedal and clutch in, revs will jump
-If you are trying to shift through the gears, revs will jump when you clutch in to shift
-Usually happens within the first 10-15 mins of driving, if it doesn't happen then, then I don't notice it for the rest of the drive.
-Might be a coincidence but seems to not happen if I turn the car off and back on?

First 2 videos from earlier today, last one was recorded just now.

Seems to retract fine, car off -
Seems to be working fine, car on -
Seems to be retracting very slowly, car off -
It's too late at night for me to try now, but I am guessing if the car was on in the 3rd video, the revs would jump/hold, correct?

Going to try and clean and lube what I can, maybe that may help before maybe replacing the cable if that's the issue.

Thanks in advance.

The cables are extremely cheap. Just replace it.

Good point. I guess I was maybe seeking some assurance that it does indeed sound like a faulty throttle cable and replacing it would fix the issue I seem to be having.

I'll place the order for a cable tomorrow as there was none at the dealerships in town today so hopefully by next week I'll know for sure.

Also nevermind all the dumb m.net threads about replacement being a pain in the ass. Look at your new cable, observe the orientation of the two tabs under the dash. Use a small vice grip to depress the tabs and push it out.

Those cables can melt if they aren't shielded from all of the exhaust components. Throttle return springs can break (I haven't specifically heard of it happening on a Miata). They can also be kinked or crushed by careless work under the hood.

As Andrew said, definitely change cable, but try to determine the root cause so it doesn't reoccur.