I solved it among with a few other problems too...
One of the bulbs is almost burnt out. It's some Taiwanese bulb (no offence) so I replaced that.
The problem, I think lies in the left taillight cables. A few of the cables were exposed!!! yikes... so I just taped 'em and I haven't been able to repeat the problem...
so all is good.
2008 BMW 335i Coupe