You were right, definitely one needs sparlplugs with 50k resistance.
I first did not believe it since there was already 50k on the ignition cable (Beru connector).
I thought that the noisy speed given by Vems through the PS2 input was not related to the USB messages and comes from a noisy GRD. I improved the GRD, made a stronger pullup on the signal, the speed comes a little better.
Then , I thought to improved the grounding of the RS232 cable, nothing ... I scoped the coils for resistive versus a no resistive plug. nothing relevant.
Then I changed the plugs... The only way to protect the Vems circuit from any electrostatic transient voltage seems to be damping the transients at the origin.
Even if it could be obvious, I believe the recommendation for resistive plugs should better be put in the documentation (if not already there ;-))
btw. Another one should be to feed the IAC from an inj output. Then idle is fast tune. If fed from ign output, one (me) spares a lot of time and may need to work out as well the VE/Ign map at the idle point.