2.5L CRD TD Power Loss

hey all,
i own a nissan navara (2008 model) 2.5L common rail turbo diesel ute which has 8800km on the clock.

first service is at 10000km.

well it happened around 8100km that the engine seemed to have lost power.

originally the first 5000km or so i ran the stock filter, but me being young i chucked a Pod filter on as it was high flow and seemed to work pretty well (i didnt take it offroad as its a 2wd).

at around 7500km i put back on the stock filter and for some reason the car ran like a bat out of hell. i had so much more torque and power and i was loving it.

though after all this enjoyment of having a car that launched you back into your seat and wouldnt allow u to drop the clutch at 1100 revs without the tyres spinning, it seems to have dissapeared.

though it happened around 7900km that when refilling with jerry cans the red jerry can lining peeled off and got it my fuel tank. it ran fine for ages until 8100km (300km later) which is when the power seems to have been lost.

because i thought it could have been the paint i replaced the fuel filter which was full of paint flakes and i cleaned out the tank by hand at 8200km as i had a road trip. i also put a whole tank of new fuel in.

through the road trip its been fine and had no issues, but it still just didnt have that insane power it had before.

so still to this day it just doesnt run the same and doesnt have that great low end wheel spinning grunt like it used to, but she still does the job and goes well.

so what does everyone reckon? whats the issue? what should i get done? what should i replace?

thanks for reading this all and all questions are welcome.

cheers,
george