Something is loose...tighten things up...learn to use the cluster test to keep an eye on things...but this sounds like a loose/corroded connection...and I hope you get lucky and that's all it is.
Start at battery...alt...+ terminal...grounds
Upcoming NYC dates: Jan 8 and 27. See Facebook for other shows and PM me if you can make one...love to see you there!
Mangofy your E46!