Shuttering/Bucking, Power Loss Between 3,500 to 5,000 RPM
I'm having power loss issues between 3,500 to 5,000 RPM. Usually once the RPMs reach 5,000 the car will eventually begin to pull. This only happens when the car is in gear and it's very irritating to have bucking and jerking when I mash the gas until 5,000 RPM when a get a moment of nice power.
I have noticed that in the cold mornings the car RPMs will sometimes jump to 2,000 RPM until the engine gets warm.
It also sounds like sometimes the engine beings to backfire.
So far I have tried the following.
Replaced the fuel filter.
Replaced the spark plugs, gapped correctly.
Replaced spark plug cables.
Replaced the manifold cup-links with the CFM ones.
Replaced fuel pressure regulator vacuum line.
Which is the most likely cause
Air leak behind the MAF sensor?
Fuel not enough fuel pressure? Although the engine runs strong above 5,000 RPM.
Thanks in advance.