On a lot of vehicles it's the ignition switch connector and not the switch itself that fails. Regardless, I think you need to learn about voltage drops and current so that you can pin down the problem when it's happening. I would say starting at the battery connections and working towards the starter.

__________________
1988 325is (purchased new) sold
2004 X5 3.0 2005 X3 2.5
2008 X5 3.0 (new to me)