It wasn't the timing belt. If the timing belt was bad, the engine wouldn't start at all. But replacing it wasn't a bad thing to do. In fact, replacing all the stuff you've mentioned is good - you've eliminated a bunch of things already and now when you've got this last problem sorted, it should run great for a good long time.
I'm guessing it's an automatic. So, put it into gear: engine slows down and dies? Your idle-up actuator isn't working. It turns up the engine a little bit to keep it running at the right speed even against the light load of the torque converter at low speeds - the torque converter allows slippage so the engine can continue to run. Lots of slippage at low speeds, so you can sit at lights without shifting out of gear, and next to no slippage at high speeds, so it works well when cruising along with traffic.
Idle-up control. What Ford calls it may vary, but that ought to be it. There's a similar dealie to add power when you turn on the AC.
