M42 cold running problem

[M42-Matt]

I am struggling to diagnose a '91 318i M42 cold running problem.  At start up, it idles fine, but backfires out the exhaust if I give it throttle.  After warming up, it runs great. 
1. I have checked the coolant and intake air temperature sensors, the resistance of those both look fine.
2. I get no CEL, but I and am seeing an occasional cam position sensor fault when I pull the fault codes.  I cannot find a new genuine part, so I have tried replacing the sensor with a new Facet aftermarket replacement, and also tried a good (I think) used genuine BMW sensor and still get the occasional code.  I checked the cam timing relative to the crank position and it looks fine (with cam holder tool on back of cams and a 5/16" pin in the flywheel recess).
3. Checked for vacuum leaks and un-metered air entry multiple times.  (The idle air and crankcase vent heaters were bypassed long ago.)
4. Compression and ignition system test good.
5. Fuel pressure and delivery rate test good.
6. Replaced O2 sensor with new Bosch part for good measure.
7. Replaced spark plugs and air filter for good measure.
8. Swapped ECU with a good used one.


I have two other good running 1991 318i's and can swap parts as needed.

Suggestions? 
Any subtleties in the cam position sensing or timing that I need to better check?
Should I buy a scanner with live data capability?  If yes, which one?

Thanks in advance.