ECM fault?

[Akselrud, Boris]

Unfortunately no one answered my first post regarding what I think is ECM
related problem in 5.7 TBI in my 1993 Cadillac Fleetwood.
The problem is erratic or I would say unstable idle. Looks like ECM is
leaning the mixture (don't know for sure) causing idle to go below
comfortable level for 2..3 seconds followed by 2..3 seconds of smooth
operation.
I've gone trough repeated sensors and parameters check on the engine during
the last two years. I am confident that all the sensors working within spec
and I scoped the output from all of them. Mechanical condition of the engine
is also good - no noices or knocks, compression tests like new, ignition
system checked repeatedly and spark plugs replaced, no vaacum leaks. Another
words the mechanical condition is not at fault. However the output of O2
sensor at idle is somewhat slow with a period of 2 seconds. I don't know if
this is normal for that engine.
Could anyone tell me if you have experienced similar behavior of the engine
at idle and if you got it corrected. Can it be ECM induced?
Please, share with me what you can, I have exausted all other sources.