1993 Mercury Sable will not accelerate, check engine light

hi. Once warmed up, this otherwise smoothly running engine will not accelerate, and stalls. Check Engine light comes on. Press the gas, it simply will not rev up or go. To move forward at all, i must barely touch the gas, and accel very very slowly. Runs rough and may stall if stopped or in Park for over a minute or two. Restarts easily, but symptoms continue. I have noticed a burning smell inside cabin if i persist in driving. Cannot locate source. Revs fine in Park, and Reverse seems OK, stangely. Black smoke present once this occurs.
Diagnostic at shop says: Idle air control valve. I replaced-- no difference. Replaced back. And here I sit. Any suggestions? thanks.