The fact it happens under load could be either ignition or fuel.

If it's the latter you need to be careful, as you will end up with a meltdown, thus I would recommend you get it setup, part of which will confirm your AFR.

I don't think misfiring is particularly a problem on your car, this is what we suffer with, i.e. Cosworth owners due to the old ignition system, with WS (i.e. what you have) it provides a stronger spark and less voltage loss etc, the exception being if something is in a poor state, but you say you have replaced such components.

Martin