could be a dodgy cam mate...mine was doing this a while ago but was between 3-4k...engine would run like it was on the soft limiter...only way to rev past it was to get into a lower gear and floor it...with a few people in and my install...nething in 4th or 5th past that rev was non existant...unless i had a downhill run lol

have you also checked the timing with a proper timing gun? try setting it to +12degrees with vacuum off and take it for a drive and see how it runs

bassboy