Hi glen

I would imagine if you are back probing the rpm sensor you may be making a better connection to the sensor.
Saying that I would expect the vehicle to cut out upon disconnecting the scope?

Have you got any rpm signal at the ecu when the vehicle doesn't start?

Will the vehicle still fire up if you scope the sensor using a wire piercing connection?

Can you use a couple of jumper wires from rpm sensor to the plug, you maybe able to ensure a better connection.

hth
James