gilera runner vxr 200 (53 plate)?

hi i have a vxr 200 and it has a flat spot at 55/60 mph i have put a new cdi unit on it, a new coil pack ,a new inline filter, a new fuel tap and had the carb stripped and clean a couple of times, i have drained the tank and re-filled it with fresh petrol, it does have an after market exhaust (scorpion i think)but i believe the jets are the right ones for the exhaust ..... please help it driving me insane lol