| I'm no expert but I had the same spluttering going on at about 5800-6000 rpm. If it's always at the same rev range it could be the throttle position sensor (TPS). This is basically just a potentiometer that tells the ECU how far the throttle is open, if the tracks are cracked or dirty, you would get these symptoms (although you're looking at a replacement TPS cos you can't really repair them)
Neil at Cornerspeed sorted mine out for me. Give him a call or ping him a U2U. | 
Jools
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