Have you checked codes for the throttle assembly? Pin 14, I think (search is your friend here).

I have the same car, and it had the same symptoms you originally posted. Turned out to be a dead MAF and aged O2 sensor. I had pin 8 code 4, and another - forget the exact code - that implicated the o2 sensor. A fresh MAF fixed code 4. Driveability was perfect, idle was improved but still a bit lumpy. A fresh O2 sensor delivered a perfect, seamless idle.

What does "falls on it's face" mean - does it not respond to the throttle at all? Or is it slow, or what?