Help!

The Problem Is Intermittent, Sometimes When You Are Driving It Starts Running Really Bad And Sometimes It Does It When You Start It But By The Time The Mechanic Can Put The Scanner On It , It Is Running Ok It Is Almost Like It Is Not Getting Fuel Or Like It Is Not Hitting On All Cylinders But The Mechanic Says The Fuel System Is Ok And The Electrical System Is Ok .
No Codes On The Scanner
The Mechanic That Has It Has Checked Fuel System No Problems Andelectrical System Is Ok It Only Does It Once In A While It Is A 1998 Chevy Lumina

Has he ever done a vacuum test on it?
My guess would be a cracked or dryrotted vacuum hose. They can cause intermittent problems. I had a dodge 600 that would run like crap some days. It would hardly move with the pedal on the floor. It was a turbo though so once the turbo got spooled up it would take off. Other days it was fine. One day it just died on me when I was on my way to school. So I had it towed to a shop my grandfather recommended me to. $17.50 later I had a new vacuum tube and a new fusible link(which is what made it die, it fried and the power to the ignition switch got cut off). And the car ran better than ever. The guy gave me the hook up cuz he was real good friends with my grandfather.