I have recently noticed that my engine doesn't pull as hard as it should, especially in the midrange. The engine sound is also less agressive, and a couple of times, when accelerating, the engine has been "jerking".

Any idea what this could be?

The car is a 04 RX-8 HP with approx. 31K on the clock.

Have the spark plugs, plug wires, and/or coils every been replaced?
How about the cat? Might want to make sure the cat isn't clogged.

If you have 31k and your car is doing this then I would say the samething Jon316G said. Why? well I just chaged mine due to the same issue. Guess what? Car feels like its brand new!

Thank you guys.

The car has been serviced regularly, and I know the spark plugs has been replaced once. I believe that both the plug wires and coils have the same age as the car.
As for the Cat, if it is clogged, would the car really respond like it does? The throttle response is also a bit worse than it should be.....

The car is to be serviced in 12 days, and hopefully the dealer will find the problem.....

It could be the "lack of power" TSB requiring a compression check.

http://www.finishlineperformance.com/pdf/rx8/bulletin/01-013-08-1925.pdf

Ok, so the car has been serviced today. I told the dealer about the problems, and when I picked up the car, they told me that a change of spark plugs most likely did the trick. When I left the dealer I was quite happy actually, but when driving the car it was clear that the problem was still there.....:rant:

Could it possibly help to clear the ECU? The engine response is worst between 5K and 6K RPM. At higher revs it pulls like it shoud.

Resetting the ECU did nothing...

I just went for a test drive, and the engine pulls like it should from idle to around 4500rpm, and from 7000rpm and up. The problem area is 4500rpm to 7000rpm. Does that indicate anything in particular?

Doesn't the second intake port open around 4500rpm and then the third just after 7000?

Maybe the second port is stuck and needs a good cleaning? I've seen this issue before when the second port gets cruddy.

//not too mechanically inclined so maybe someone could confirm this?

Doesn't the second intake port open around 4500rpm and then the third just after 7000?

Maybe the second port is stuck and needs a good cleaning? I've seen this issue before when the second port gets cruddy.

//not too mechanically inclined so maybe someone could confirm this?

Your explanation may very well be correct, and after really pushing the engine for a couple of days (high RPMS and hard accelarations), it has "loosened up". I have always driven my car at high RPMs, but maybe it needs even more high RPM driving? :)

If the problem still seems to be around or comes back, you might want to check this thread if you haven't already
http://www.rx8club.com/showthread.php?t=165478&page=1

Ok, so the car has been serviced today. I told the dealer about the problems, and when I picked up the car, they told me that a change of spark plugs most likely did the trick. When I left the dealer I was quite happy actually, but when driving the car it was clear that the problem was still there.....:rant:

Could it possibly help to clear the ECU? The engine response is worst between 5K and 6K RPM. At higher revs it pulls like it shoud.

Hey man, I had this same problem a while ago. I bought 4 new coils, and drives like new again. Just make sure when you swap them, do one at a time so you don't mess up the connections. I have an 04 too and had about 38k when this happened.

It could help to clear the ECU, but it's unlikely. It's free though, so probably worth a try (I cleared mine at least 10x in trying to diagnose my problems.) If you don't know how, put the key in, turn it to run (don't start it,) and tap the brake pedal rapidly 20 times until the oil pressure gage swings up and back.
I agree that coils are a good next step, plugs usually go bad on this car quick because the coils pushing them get weak. The coils have been improved since '04, and there are aftermarket kits available that should last forever.