I'll just update the initial post since I wasn't aware of the catalytic converter thing, my bad!

2000 XJ Sport, 4.0 L6; rebuilt the top end ~10,000 mile ago and the following parts were all replaced with new parts: cylinder head (everything in it was new as well), cam, lifters, push rods, rocker arms, water pump, thermostat, oil pump, timing chain, and the stock single-spray injectors were replaced with some OEM quad-sprayers.. I think that's everything.

Sudden power loss and rumble at certain RPMs (seems to be about the point the overdrive should be kicking in), ranging from a low rumble and slight vibration during acceleration, to a much louder noise if I stab the gas. Going up hills causes a noticeable power loss. Outside of those conditions, it runs fine and is actually more quiet now that I've changed the oil than it has been in a while.

Strangely enough, on the way back from town today, it did none of this, even going up a slight hill on the normal path back to the house. When it was acting strangely, though, on the way to town, another thing I noticed is that if the A/C is running, the blower also seems to lose power and cuts air flow from the vents seemingly in half. That bit lasted for ~15 seconds and then picked back up to normal amounts of air flow. (I hadn't noticed this before.) The motor also seemed to stumble quite a bit, when I hadn't noticed it do that before.

One thing that I know is likely suspect and needs replacing is the fuel pump. It sometimes does not quietly hum when you turn the key on, and instead sounds like it.. knocks.. or something. Probably not coincidentally, whenever it makes that terrible noise, the engine won't fire and I have to try again.

Is it possible that the fuel pump has gotten bad enough that it could be the sole cause? I also noticed today that the engine stumbled quite a bit, which I also hadn't noticed before. The fuel pump was one of the few things I haven't replaced in the thing, and it is almost 20 years old and acting up anyway.. so I know it at least needs that.

 

You're not supposed to discuss removing the cat on this forum, did you know that? You might try disconnecting it and see how it performs however, hint, hint.

 

I actually wasn't aware, sorry! I went ahead and just updated the initial post entirely, haha.