2003 malibu 3.1 jerking/serging?????

My wife's malibu seems to jerk or serge when at 60-70 kilometers per hour. I've watched the tach when it does this and there is no fluctuation so I believe it might have something to do with the tranny shifting. Seems it might be when it's shifting from 3rd to overdrive. Any ideas what to look for or what it might be?

Thanks