the cat failing can do that, somewhere or other i have the bit from inside my dads old cat, it had broken down into what can only be described as an egg shape and at certain revs it was pushed back into the exit of the CAT blocking it, engine could slowly push past this point if you kept your foot planted but didn't like it.
I remember almost exactly the same thing happened to one of the cinq turbo's on here, was a while back when martin owned a yellow one but its not really been on the forum for a long time now.
Eklipze3k's sei is doing this exact thing atm, gets stuck at 3k and i'm pretty sure its a blocked silencer on there (no CAT fitted).
I'm not sayiing its defo the problem but it could be and i have seen it more than once.