2000 Hyundai elantra

I just got this car on the 2 of April 5 days later I was having problems with it. when I test drove it, it was perfect and then out of no where when I was hitting 20 mph second gear it would slip and rev up a Lil I would have to let off the gas then it would shift into gear. could it be the solenoid shift chip. the car lot took it back claimed that they changed the filter and fluid and then it did stop but now its doing it again. What could it be?