Ignition switch is probably on its way out.

Mine did the same thing when the voltage got down to 12.2 or lower. Above 12.2 and it fired up everytime.

The only strange part is what normally happens is it will do nothing when you try to start it when messing up so you may just have a loose wire at the starter instead.

Hard to diagnose stuff like that remotely over the net and all.

I would check for loose starter wires first and if they are fine I would do the ignition switch if it was me.

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Aftermarket E53 Radio Install