Higher Octane is not per se better.
The Jalpa and Countach are designed for 98-100 Octane (European Spec.) whereas the Diablo runs with 95 Octane.
The higher Octane rating means a lower self combustion, therefore the lower knocking ability, but this means also it needs more ignition power to start combustion at all. If your ignition system is weak it might possibly have problem to ignate a high Octane fuel mixture at the correct point.
I have to admit I have no idea at which octane rate this might occur... I think a bad mixture (lean or rich) has a higher influence.
If you use 98 Octane Euro Spec. you are on the safe side with the Jalpa and Countach, I also used 95 without any problems. My MSD-Ignition burns almost every mixture
Raymond