Hi,
This hapenned to me with my wagon. It was sporadic and unpatterned. The problem was a loose ground wire. It wasn't the battery ground but one related to the transmission. apparently a common problem as I learned of it here or on another forum. I don't wish to diclose the embarrassingly large sum of HE (hard earned) cash my very well-off, former and inept mechanic realised chasing this month by month. I should add that he is a graduate of MB training. Another mechanic fixed it in 15 minutes. I'm sure this is your problem.