1998 Chevrolet Silverado

Other Category problem
1998 Chevy Silverado V8 Four Wheel Drive Automatic 81,000 miles

I recently purchased a 1998 Chevy Silverado. It has been running great, except yesterday it started missing out (just a little), idling lower than normal (around 500 RPM), surging while driving and now the service engine soon light has come on. I popped the gas cap off, put it back on and the light went off. The light has since come back on & I am left wondering what is going on! The previous owner filled it with cheaper gas (85 grade) for the first time the day I bought it and I am wondering if that could have something to do with it. And ideas or suggestions?
Thanks for your help!
December 4, 2007.

When was the last major tune-up?

Could be multiple problems-for the idle and surging check the idle air control valve and throttle position sensor.

Gas cap-have the Evap system check

Dec 4, 2007.