I just cured a customers car of the same problem - in fact it would stall returning to idle as it stopped at a light.
I suspected a weak mixture, and it turned out the idle air bypass hoses were slightly loose on the auxiliary air valve, and to the idle adjuster. I replaced a few, and put clamps on the rest. These hoses seemed to shift around and add air - every time I had the air cleaner off it idled differently.