2000 has always been down on power

Hey guys, I've had my 2000 XJ for quite a while now, 4 years and almost 50k miles. Its always been down on power compared to other XJ's. I put on a Spectre cowl intake, full 2.5 exhaust, 2.5 High Flow cat, 62mm throttle body, injectors, as well as doing plugs to make sure there's no issues. While this helped a bit, its still a dog. A friends 99 on stock 3.55 gearing and 33's has more power. Where would you start looking to help it pick up some power? About once a year it pulls a code for an O2 sensor then goes away. Thanks for any help!

All 5 of my other 97-99 Cherokees have run circles around my 2000. The 2000-01 0331 head has smaller exhaust ports. It seems as if the power doesn't come on until it reaches about 2000 rpms. If local smog laws allow, you could look into swapping out the 0331 head. You may have to make some adaptations/changes to the exhaust manifold and down pipe. You will want to keep all the 2000 emissions equipment, and adapt it to function correctly.

Yeah, I havent cracked my 0331 head yet as far as I know. Knock on wood. My coolant has always been a disgusting brown with lots of sediment. I suspect this is mostly because of block rust, but who knows it may be cracked.

Easy to check. Make sure you're full up on coolant, get the motor warm (operating temp), shut it down and pop off the oil filler cap and stick a flashlight in there. If it's cracked, you should see tiny green bubbles coming out about 1" toward the firewall. It may not be super obvious, and you probably can't actually see the crack as it's microscopic, but dirty coolant with the 0331 strongly suggests a crack. Just found it on the 2000 I just picked up for the girl. Didn't notice till it was running hot and I could hear a slight hiss from the valve cover.