PWM Idle disturbs RPM

[mollama]

Hi,
i use 029q with a 4+1 trigger wheel, and i tested the PWM idle feature.
I use the Volkswagen original Valve (Cigar) its a 2 wired.
the valve works but if i activate the feature the RPM jumps intermittent to 8000 or more, but only for one or two cycles.
what can be the problem? should i use shilded cabels?

thanks Thomas