Help needed: Code 42 - Electronic Spark Control Error

[QB93Z]

We are trouble shooting a Code 42 - Electronic Spark Control Error on a 1991 ZR-1 that was rebuilt after a cam chain failure.

I don't know if this problem is related to an idle speed control problem that we are also trouble shooting. The symptom of that are that the idle rpm oscillates from ~950 to ~1100 with about a 10 second period. During the oscillations, spark advance measured by a Tech 1 is also oscillating.

The Service Manual indicates that Code 42 signifies that the ECM is not seeing the correct voltage on the EST signal from the Ignition Module. After working through the trouble shooting flow chart, the problem may be a failed Ignition Module. Based on how rare IM failures are, I am looking for suggestions of other things to look at. We did verify circuit continuity and connector condition which are other possible causes of Code 42.

The Code 42 can be cleared by the Tech 1, but is set again as soon as the engine starts.

Has anyone had experience with Code 42?