2004 S10 Surging at about 40 mph

My husband has a 2004 Chevy S10 4.3l 4wd with only 35,000 miles. At first when the problem started it would surge when he went around corners at 40 mph. Now it surges all the time at or around that speed. It's like he is pushing on the gas and letting up even though he is not touching it. Also, it winds up really high when he tries to speed up quickly. Can anyone give us some advice?

If you turn the key to the RUN position, before you start the engine, does the SES, (Service Engine Soon) light come on? If it does, after you start the engine, does the SES light turn off?

Yes to both questions.

Has the fuel filter been replaced recently? How about the distributor cap and rotor?