2005 Chevy Malibu feel a "slip" when crusing or m

Hi guys,

Last couple weeks driving my malibu maxx I've noticed a subtle quick slip mostly when on cruise control around 50-60 mile and sometimes at lower speeds. I thought at first that it was transmission because it was happening when the car was shifting gears but then I started noticing the slip when the car wasn't shifting. I took it to a shop and they couldn't find anything wrong with the transmission, and they couldn't feel it on their test drive, but I definitely feel it. The engine light hasn't come on. Is this a cause for concern, what do you think the problem could be? This car has records for oil changes every 4000 miles since first bought. Thanks! -Nick