1994 Buick Century EGR valve control solenoid?

I'm not sure, but it seems to me the most likely way the EGR would cause stalling at idle is if the valve itself is stuck open. But that would probably happen when the engine is cold, too, and you said this happens at operating temperature. I imagine the solenoid would more likely cause the opposite problem, i.E, not allowing the EGR to open sufficiently, which might cause a check engine light and probably lower fuel economy but not driveability problems. At any rate, the solenoid should have a positive terminal you can at least check for voltage.