2013 Silverado RPMs jumping at steady speeds
 

My truck is only six months old with less than 5,000 miles. Recently the RPMs have started jumping about 200 RPMs when driving at a consistent speed. This occurs about every 20-30 seconds. I took it to the dealer and left it for two days for them to tell me there's nothing wrong with it. Very frustrating!! Anyone else out there having this problem with the new 6 speed Flex Fuel models? I see its common in older models. Help!!