Is It A 4 Banger? I've Had The Same Trouble; I Am Preparing To Swap The Throttle Body On My '90 Lumina 2.5l. It Has Been To Several Mechanics And Had Every Ignition Part Possible Replaced, To No Avail. It Was Finally Properly Tested, And Determined That The Fuel Injector And Fuel Pressure Regulator Were Faulty.
Have Those Tested, And Also Check The Egr Valve And Passages.