Bucking 2002 Saab 5spd - 141K miles

Hi, About half dozen times, randomly, this past week, driving 60mph on highway, I hear/feel a quick clunk - feels like the car bucks for an instant, then goes along fine. Doesn’t seem to happen around town, but then again, I might not notice, as I am stopping/starting in city traffic. Bought car used at 85K miles; have never had work done on clutch or transmission, so worry it may be this. Any advice appreciated.