sounds like tune up time to me, check and replace if necessary spark plugs, spark plug wires, cap and rotor, fuel filter, air filter, see if the problem persists if it does do a compression check and see if one of the cylinder's compression rates are off from the other 3 could be uve got a vacume leak or blown gasket somewhere. but always start with the basics first and work ur way to the more complicated problems could just be crappy spark plugs or bad gas. and what are u idling at?