As long as you are using O.S F plug and a nitro/oil content according to instructions, it should be OK.

Since the idle is reliable and WOT RPM readings are within normal limits, it sounds like you are having a pre-ignition problem. Check your camshaft timing. I have experienced it on one Saito 80 with the same problem. With wrong timing, it can develop extensive vibrations above 1/2 trottle. This timing issue can also be the reason for the poor fuel draw!