1999 Buick Skylark will hesitate or surge

1999 Buick Skylark mileage: 76,000. When the engine warms up to normal operating temperature, it will hesitate or surge when the throttle is depressed. It will do this when it is in drive or at idle. You can make the engine really stumble by moving the accelerator on the engine at the throttle body. No readable error codes in the computer at this time. When this happened before (April 99), the dealer replaced the mass airflow sensor, replaced the computer chip with an updated version and said the injectors were dirty, so they added some high strength cleaner to the gas. The hesitation stopped after that, but now it is back again. Any suggestions besides taking the car back to them. I have checked all the obvious things.