2004 volvo s40 automatic 4-cylinder 1.9L turbo acceleration issue
 

Took the car today for a ride and as I was accelerating from a stop sign, then it felt as if the car did not catch the gear because the engine just revved up.
As soon as I let go of the gas and applied slight pressure back on the gas pedal it worked fine again.
It's an automatic and this is my first experience of this happening. Should I be worried or is it just a fluke. What could make this happen?