It could be many things. The first thing I would ask is the last time you had a tune up. How old are the spark plugs, wires, cap and rotor?
There could also be a problem with the vehicle not getting enough fuel or it's getting too much air.
You should have the car serviced before the problem gets worse.
Take it in to your local dealer or to an ASE certified technician. You can find a certified tech near you by going here:
〉 Answered on Feb 7th, 2007 by Lori Johnson, Owner and Instructor at Ladies Start Your Engines