95 Grand Am SE v6 acceleration problem

i have a 95 Grand AM with a quirk when acceleration. At right around 55 MPH when doing a slow acceleration i experience a sort of flat spot. I don't lose power, the engine doesn't race just seems to give a slight bucking sensation. If i continue to press on gas as speed passes 55 it is fine. It is just a momentary "flat spot". Same thing occurs around 75 mph.
Someone told me it might be a Throttle sensor. Had a mechanic check it out. Not throwing any codes. He got same results on road test. He thinks it might be symtom of Torque convertor starting to go. Suggests waiting to see if it starts to happen more frequently at lower speeds so that it might be easier to diagnose.
Does anyone have any ideas what it might be. My son uses the car for school and I don't want it to break down while he's on the highway.

hey bob98, hav you checked the fluid level or if its smelling funny and brown, I'd change teh Fluid&filter ,Might just need that changed. vbmenu_register("postmenu_3736303", true);