1998 Ford Contour stall? Between 55- 65

I just bought this car 98 ford contour sport with the 2.5 v6 auto, I was told that the transmission was rebuilt with heavy bands and a new torque converter. I just noticed now that sometimes very randomly it the car while cruising seems like the tranny almost slips and the rpms go from 2 to 3 grand all on its own. What would be the cause of that? Like I said its very random you let off the gas and it may or may not go away, it will jump rpms and then correct itself. Any ideas? Thanks