2003 3.4 chevy impala

I have a 2003 chevy impala with 140,000 miles on it.. it runs great. last week when i was driving it on the freeway i was going 60mph then traffic slowed. when i went to gave it alittle gas i felt the car accelerate then i felt the car accelerate in and out. i let off the gas and gave it more gas then it was fine. it happened like 3 more since then. no lights are on or anything. what could it be... can anyone help.......