Router spindle electrical noise.

The electrical noise generated by my Porter cable Router is
triggering my limit switch circuit. The limits are high and go low
if there is a problem. They are all in series (NC). If I set the
speed on my router down 20% or so the problem goes away. I slid one
of those ferrite donuts over the limit switch cable with no effect.
Is there something else I can try? I used shielded cable on all
cabling EXCEPT the limit switches...do I need to replace all that
cable? Are there other options?

Thanks for any help you can provide.

Eric