The car hesitates / lacks power until about 3000 rpms then picks up. Have changed coils, plugs and wires as it seems like a misfire at times. Engine revs fine while not under load. Occasionally will hesitate or act like a misfire at idle too. engine is the 3.4

Could be fuel related. Have you scanned it for codes ?

If your plugs are good on those quad engine i change lots of ignition coil and the cover plastic crack and the spark jump check the video

P0300 - Random, Multiple Cylinder Misfire Detected Fault Code Definition

• Random, Multiple Misfire Detected

Symptoms

• Check Engine Light flashing
• Rough running, hesitation, and/or jerking when accelerating
• In some cases, there may be performance problems, such as dying at stop signs or rough idling, hesitation, misfires or lack of power (especially during acceleration), and a decrease in fuel economy

Common Problems That Trigger theP0300

• Worn out spark plugs, ignition wires, coil(s), distributor cap and rotor (when applicable)
• Incorrect ignition timing
• Vacuum leak(s)
• Low or weak fuel pressure
• Improperly functioning EGR system
• Defective Mass Air Flow Sensor
• Defective Crankshaft and/or Camshaft Sensor
• Defective Throttle Position Sensor
• Mechanical engine problems (i.e.-low compression, leaking head gasket(s), or valve problems

There are a few things that can cause an engine misfire and hesitation. Does your van have a CoP ignition system? (Separate ignition coils for each spark plug) Or coil pack? You may have bad ignition coil(s); that would cause a misfire. If you have a dirty/clogged/leaky fuel injector for cylinder 3, that could also cause a misfire condition. When you replaced all the spark plugs, did you look at the plug from cylinder 3? Did they all look the same? Sometimes you can get clues to what is going on in your cylinder by how the spark plug looks: if it's white or glazed you are running too lean and could indicate fuel injection problem, if it's got burnt oil on it and it's sludgy you may have a leaky gasket or worn piston ring that is letting oil up into the combustion chamber and fouling the plug, causing a misfire. If your check engine light comes back on and it's the same misfire code, have the coil for cylinder 3 checked to see if it's weak or bad. Or you can do a home remedy check and swap a known good ignition coil from another cylinder to 3, reset the code, and see if the misfire moves to another cylinder. If it does, you know the coil is bad. You can pick up an ignition coil for pretty cheap on the internet ($30-$70) if that's the problem and they're simple to install (a pain if they're on a cylinder that's under the intake manifold, but just because you have to unbolt a bunch of stuff to get to it...time consuming but not too difficult). I have a 2001 Mazda Tribute and had a couple of ignition coils go bad, causing misfires. This seems to be a semi-common problem for early 2000's Mazdas.

cylinder 4 misfire ,now if this is a GM product then i think you may need a reconditioned ECU init as if this is the 4 cyl engine and its same as the opel astra and zafiro and the vectra then its a common fault with a 2000 to 2004/5 model the computer is bolted to the engine inlet manifold and very weak and susceptable

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

Try changing spark plug wires and have the fuel pump checked out and check fuel filters and fuel line to see if that is your problem.

Could be the other coil pack. Be sure to check and clear all codes stored.

First,investigate the warranty,as there is no need for you to be spending money out of your pocket if the warranty covers such items.As far as the misfires,they can be caused by items a little more in depth,although the spark plugs are the first items to check and replace.The spark plug wires can also cause misfires,as well as coil or coils,and injectors being dirty and partially plugged up.If your engine utilizes spark plug wires,replace them,as they will leak voltage when old,and cause misfires.Anything beyond that will require more diagnostics.Every diagnostic trouble code (DTC) has a diagnostic route as prescribed by the manufacturer.These diagnostic routes are available through pay sources such as All Data online.If the simple things like plugs and wires doesnt fix the problem,then diagnostics are in order.Good luck.

Did you try replacing the spark plug wires? They might be old/ frayed/ crimped. Otherwise check your intake valves for correct compression and sealing. A bad seal can cause lack in power at low RPMs.