how can i fix my aprilia rs 125?

hi there
i have an aprilia rs 125 2003 the past couple of week i have been having problems with it it starts fine an pulls off fine but from 5000 rpm to 8000rpm its spulttering an sound ruf the brought a hayns manual read throu it an from that is sound like its my power vavle. i have taken the power vavle apart cleaned it all up the oil seal is ok an so is the rest of it. i am getting the right power down to it from the ecu the cable is fine it has 0.5mm play witch what the manual say it should have. i have cleaned out the carb tuk it apart gave it a blast throu with some wd40 all is ok but my bike is still doing the same an i am at a loss can you help me please.