[Greenless]

the black smoke is becuase you are not completely burning the gas, and when you combine that with a a bad idle i would guess one or more cylinders are missfiring from either bad plugs or bad timing. im no expert thats my rookie diagnostic, check your dizzy, your plugs, and your wires.

 

[Greenless]

If the problem is erratic, then I would suspect wires, cap, rotor, plugs, or dirty fuel injector. If it was a temp sensor or some other device it would run bad all the time after it warmed up. Still could be a temp sensor, but I would check the other items first.

but a short or break in a wire, or abad ground might cause it to flop sometimes, but others go back to normal, if thats the case expect it to get worse with time.

i had a bad idle when i bought my car and couldnt figure it out, tried cleaning the maf, checking grounds etc. then i changed the t stat and it fixed it and has been fixed for over 8k miles. all the temp sensors are integrated in a way that when one is bad they all act up