904 slipping

today we took the car out for a short drive and found a flat straight road. we stoped and took off like we are on a track..at about 55 to 60 it felt like the trans slipped so we let off the gas.. went back to the house and checked the the oil and it was just a little below the full line.. during normal driving it drives great no issue before we put this trans back into the car we took it to a trans shop and had them look at it and everything checked out so we put it back in.. we rebuilt the trans a few years ago and jut want to make sure it was proper still. what would cause the trans to feel like it slips. it 3500 stall 904 trans. want to figure this out before we take it to the track