91 mazda 626 erractic idle surge when engine is cold

My '91 626 did the same thing. I tested the throttle position sensor, and it was good so that wasn't it. The other likely culprit is the Idle Speed Control valve, or Idle Air Control valve, which is a $500 part new (more than I paid for the whole car). I pulled one off a 626 at the salvage yard and got it for 3 bucks, put it on (it's an easy DIY, just 4 philips screws holding it to the intake manifold), however I can't be sure that solved the problem, since I haven't run the car much since then. It would seem the most likely cause, tho, and a cheap fix if you go used. Other possibilities: O2 sensor or Mass Air Flow sensor?