[Jamie '92 968]

Your presumptions:
Correct - Pushing the accelerator near the floor, trying to accelerate
Correct - though it seems easier to touch it off when in a taller gear (4th, 5th, 6th) where the RPMs don't change as quickly when the accelerator is down, and the shudder is therefore sustained.
Correct - no noises that I have detected

Questions:
Yes - stock
No slipping

This morning on the way to work there was no sign of it, though I tried many times. I even tried re-enacting at exactly the same place as on Friday. I think the ambient temp was even about the same both days.

I may look in the Yahoo yellow pages for an exorcist.