How to diagnose spark plug wires (inteferences?)

[DaniTD]

So, after replacing my AFM now I don't have any flat spots when accelerating. But, I still have a somewhat low idle (M40 is supposed to idle at about 800+-40rpm, mine is idling at 700rpm). And you can feel some vibrations and like a misfire every certain seconds.
So I though of the ignition system. I checked visually spark plug cables and didn't find any noticeable crack.
But today happened something strange. I was inspecting the engine compartment while the engine was running. I used a LED workshop flashlight to see better everything. And when I was checking the valve cover, the flashlight shut off. I though it was broken or loose, but every time I placed the flashlight on the same place, it shut off. Tried to place it on the same place with engine stopped and flashlight worked ok. Tried again with engine running and flashlight stopped working. It seemed to happen when placing the flashlight near the cable that connects coil to distributor cap.

Could it be some sort of voltage loss on the wires? How can I properly check them?
I remember some months ago on a long trip, with a great rain, the car started to jolt like some kind of electrical cutoff on the ignition because everything else worked fine. I stopped under a bridge and the car was idling rough. Opened the hood and there was some sprayed water everywhere, that cable had some drops too. After some minutes, engine started to idle fine again.

Thanks.

[Speedtouch]

The HT leads can give off some electromagnetic radiation, which may explain the flashlight interference.

However, it shouldn't be excessive.

Check the leads with the engine running in the dark, and see if there's any signs of tracking (sparking between lead and any neighbouring metal parts).

You may need to replace them if the insulation is breaking down.