The ignition circuit is totally seperate from the PTO circuits, they have absolutely nothing in common except both using the mower body for ground connection.
When you turn on the blades you put a load on the engine and the governor allows the throttle to open wider.
It also puts an electrical load on the engine.
it also sets up vibrations.
Now as it only happens with the blade turning, the first place to look is at the blade system.
Check that all of the blades & pulleys turn freely.
With the belt disconnected you should be able to spin the blade with your little finger.
Start with the blades & deck because it is the easiest bit to check.