Could it be the ecm or coil

There is no doubt that intermittent problems are the most difficult to find, no matter what the "subject." I've worked on or at amateur and commercial radio gear, telco gear, HVAC/R, and auto electric systems. No matter what, a "sometime" problem is always the hardest, and can thwart some of the best minds.

That is ONE advantage of the newer OBD-2 IFI systems -- so long as you get at tech that's smart enough to read the results!!!!