I found the solution if anyone else has this problem.

The lamps are held in place (when tightened) by cheap clips holding onto either side of the base. If the festoon bulbs are slightly too thick (in diameter) or the glass tube is too long, when the screws are tightened, they cause the clips to open up and lose contact with the lamp base and result in intermittant outages.

The problem happened for me when I replaced with generic festoon lamps. I suppose the clips were designed around the dimensions of a specific Bosch (or whatever) lamp.

I was able to replace with generics that were a slightly smaller size.

Bottom line, it's a cheap design, and disappointing in an MB.