Why does my 1995 Buick Century stop and running rough?

Out mechanic changed the ERG valve and the car has not run properly since. He tried numerous things and can't figure what is causing the problem.

It will run for awhile then stall-out. When I try to restart, it doesn't want to start right away.

When it does start it runs really rough and****** while drving.
He tested the computer but said it is fine.

What should we do?