[Xack]

I have had a bad switch from a good supplier. The problem I found was similar in that the key switch would break ignition between the start and "on". The car would crank, start and when I released the "start", it would shut off if I was not quick to release the key. I had to let the spring flip the key by releasing it. I replaced the switch and it's fine. It sounds like you've just got a bad switch that has bent a prong inside the mechanism and now it bridges the "Start" when you switch from other positions.

It sounds like the problem happens when you use the switch. You can also try to temporarily wire 2 switches to substitute your ingition key switch. Ignition and start separately. This way you can see if the same action happens when you turn on the ignition and then "start". If it's not the switch then the same sort of problem should be duplicated in some fashion. Or, just change the switch with a new one.