Econoline stalls

I replaced the idle air control valve, thought it was fixed, but it is stalling again. When i come to a stop, and more frequently when I shift ton reverse:banghead:

It could be lots of things.

Tune-up including plugs and wires (depending on engine)
Dirty Throttle body.
Fuel filter or pump pressure.

What year and engine is it?

mine did the same thing. it turned out to need a tranny service. the filter drops down allittle when you stop or shift gears and loses pressure. when you give it gas it sucks the filter back into place.