Have a 2004 impala 38 I think its spark plug wires but it started hesitating and at about 25 to 30 mph my check eng light flashes off and on, and it sputters really bad, but as it is sitting at idle it sounds good.

Cylinders are probably misfiring. Plugs probably need changed. Also have a local parts store check for any check engine light codes. May have multiple cylinders misfiring. Could possibly be plug wires (If it has them) or the Coils on the plugs (if it is set up like this instead of plug wires).

It could be bad spark plugs ,Or spark plug wires, Or fuel injecters, Or the motor has seen it,s better days, Depending on the mileledge on the vehicle, End if it was ever mantained (Properly)

are you absolutely certain you have the right spark plug wires to the right cylinders? Did you re-inspect the spark plugs?

tune up, maintenance and likely an air filter required
Most new vehiclwes have a hesitation but if it is a definite lag then try the above

Engine is misfiring, common causes are spark plugs and spark plug wires, could also be a coil, but plugs and wires are the least expensive place to start.

If you are sure its not the plugs or wires, I woud go with the coils next, test each one, if they check out ok then perhaps the injectors.

probably neither. You should do a compression check, and check your transmission fluid for dicoloration and metal shavings.

When did the spark plugs last get changed? May need new spark plug wires.

your tranny is slipping

You should definitely replace the fuel filter though this probably won't fix your hesitation. It wouldn't hurt to replace the spark plugs this is usually the culprit on these engine. I have the same engine in a '99 caravan and found the for some reason the spark plugs on cylinder 3 and 4 loosen up after one year and cause a misfire between 45 and 50 mph under load (when mildly accelerating).