jerking 91 Calais

I have a 91 cutlass calais 2.5L engine, 5 speed transmission. It's never given me any problems until now. I'll be driving and it will be running fine and just all of a sudden jerk and start to sputter and so I downshift and then its fine. What could be causing this problem?

i have a 89 calais and mine just started doing something similar i was driving down the freeway and it jerked then i lost most of the power i opened the hood and it seemed to be running on all cylinders but when i reved it seemed like it was only running on two or three cylinders .if you find out what the problem is please let me know [email protected] good luck