Have you tried checking/testing the IAC valve (even though it's supposed to be new)? I know there's a whole diagnostic routine for this on the ford cd.
Other than that, I don't know how else to test your IAC. You can get a ford cd by pm-ing todras.
Are you burning oil at all? FWIW, I was having the same problem and it appears to have been related to an oil pan leak that I was having. My IAC, TB, intake, UIM & LIM all tested OK. My theory is that on hard deceleration, with the TB closed, the IAC was sucking in all the "burned-oil" air in an attempt to maintain idle. Since the oil-smoke is mainly CO2 & such, the engine was literally choking. I can't back this up with any hard evidence, though.
*HOWEVER* I have since replaced my oil pan have haven't experienced the problem again. BTW, that day, the car even stalled pulling in to my mechanic's garage.
