When I bought my X5 someone had wired the power in for the left side by connecting it to the right so I was getting similar symptoms and it was not enough juice to fire up 1 and when it did it would draw enough to delay or keep off the other side. (The wonders of shade tree mechanics where ther aren't trees)

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"When the Team Chief said.... You're trapped in a hole with nothing but a goat and a slinky, what do you do? Stubby said, I'm not sure but it won't end well for the goat...." ~(Overheard) Last day, Phase 3, Q Course