Hey scotty my 2011 scion tc 6 speed 150k miles has had this problem for a while, here is a video by another person, but this is exactly what my car does https://youtu.be/tCkWdl0gwj8  

Tachometer is almost never still In idle especially when the engine is warmed up, sometimes it will be very smooth, sometimes rpms will drop that it shutters the car but most of times needle will Always be dancing around, of course when the rpms get to low the engine will make the rpms jump back up and try to compensate, car had a bad thermostat code about 1 1/2months ago But i replaced it and no codes at all anymore, I even had it taken to a shop to be inspected and have an intake and fuel system pressure cleaned but car still did the same thing. I have read on other people having the same problem, but no one seems to have a real answer almost seems like a flaw in the 2ar fe engine 2.5 but i did try New ignition coils, spark plugs, new thermostat just installed and pcv hose as well as pcv valve, there is nothing more i can do, only guess i have left is that my vehicle does have an aftermarket exhaust and at the metal exhaust tip i see black carbon buildup, alot of it, could the problem be im running too rich? I dont want to go out and buy oxygen sensors or something and it not be the problem especially since i get no codes, so how could I really know. Idle is even less still when ac is on .I just started using 93 octane, it seems to have helped just a bit but not huge.

also have a cold air intake, And no vaccum leaks have been found, thank you so much!