I just went thru this type of problem on my '87 635 and it was the main relay located on the firewall. passengers side.Also check the fuel pump relay located on the fuse box side. If either of these is going bad it can cause all kinds of intermittent problems. I found the problem after a lot of time searching and since then I have replaced all the critical relays on all three of my cars. My $.02, for what it's worth!