Hmm. Ususally when I have MAF problems Idle is the only thing is does worth a damn.
I get the feeling from all this that maybe this is a not so common problem causing a fairly common syptom.
I'm trying to think to myself "what all is happening when that idle drops?"
The IAC motor moves,
timing is adjusted,
manifold pressure rises moderately, (but this car is barometer anyway)
If the 02's were bad then the problem would be more persistant most likely.
It really seems like the most likey source of the problem is either a vaccuum leak, or a problematic ECM.
The FPR could be stuck open, but my instincts doubt it.
Get a can or two of some o2 safe TB cleaner;
First clean out the intake really really good, be careful with your MAF,
but you want that clean as well. (I do it with the TB cleaner, I dont think that is really reccomended)
Then, twist your idle screw up so it will stay running, and spray the cleaner at all the different vaccuum junctions around the engine.
If the idle is affected when you spray a certain area, then you have found a vaccuum leak.
I hope some of this is useful. Those 2.3's can be your best friend or your worst enemy.
That's why I love my 5.0 T-bird. So far it hasn't thrown me a single problem I couldn't figure out.
I never fully understood my mustangs attitude.
Good Luck!