Engine Stumble

Has Anyone Had An "engine Stumble" Problem ? My '04 Xfire Sometimes

Stumbled, Loped Or The Engine Seemed To Want To Stall But Never Really Died

When I Stopped At A Red Light Or Stop Sign .when I Took It Back To The Dealer

They Couldn't Find Anything Wrong With It And Suggested I Run The Middle

Grade Gas Instead Of Premium Gas. I Live 5000 Feet Above Sea Level In New

Mexico. The Middle Grade Gas Helped, But It Stilled Stumble Occasionally.

Now The Dealer Tells Me To Run The Lowest Grade Chevron Gas, 86%, And

They Gave Me A Bottle Of Injector Cleaner.

This Was Yesterday , Now I'm Just Testing It And Waiting To See If This

Solves The Problem. Any Comments Or Sugestions ? Thanx.