Hey Scotty, absolutely love your videos and what they have done for me to maintain my own cars as a teenager!

I have a 2016 Scion tC with 105,000 miles on it. Engine and transmission both run great (as expected from a Toyota) but I do have one recurring issue— when stopped/idling my RPMs will periodically drop down to 500-600 causing the vehicle to rattle and stutter before jumping back up to a smooth 700-800. No check engine light has ever been triggered. It has never stalled while doing this but it always feels like it’s trying to. I took it to get inspected along with doing my own research right upon discovering this problem but neither mechanics nor online forums could give me a definitive cause. I changed my spark plugs, then my ignition coils, then I did a fuel system treatment and cleaned my throttle body, all to just have the same problem come back. I even changed my transmission fluid at 95,000 miles and added Lubeguard ATF protectant after discovering that the rpm drop while idling would virtually stop if I put the car in Park or Neutral, leading me to infer that it had something to do with the transmission. But still no improvements to this annoying symptom!

I’ve had this car since 90,000 miles and I love the way it drives and handles but this one problem is irking me to the core, and I just can seem to crack it. Is this something that I should just forget about and leave be since my car runs like a dream otherwise and no codes have been tripped? Or could it be indicative of some underlying issue that may worsen, whether mechanical or ECU related? 

A millions thanks!