Well you had problems with that sensor before didn't you? I'm inclinded to think maybe it has a problem with the sensor or possibly the wiring adding some resitance.
Seems to me that once the cone is on it's still going to be pulling in air from the same place once the RPM'S are up. I cant really see how at high RPMS that air moving around in the engine compartment would overcome when it's tying to suck 300cfm in that little hole. I could see that being the problem at low rpms where it doesn't have that soret of air flow going through it.. Then there is things like the ARB snorkles and it would create a ram air effect and not heard any complaints about it. I been wrong about stuff before so I guess we will see. Easy enough to tell once that filters on. Hope it works for sake of a easy fix.
My bet is still the speed sensor. Extended RPMs with load but showing zero MPH might make it mad.
