I had checked all the obvious items, as stated above, but after testing, it was determined that the field coils (windings) were "breaking down" (meaning that the wound wire was shorting on itself and not isolated from begining to end).
I replaced it with a commercially available "gear reduction" starter. Now the car cranks and starts so fast that it will startle you!
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57 Healey BN6L-942 Wine Red
61 McCulloch R1 Yellow
99 BMW M Rdstr Cosmos Black (formerly supercharged)
01 BMW M Rdstr Steel Gray
08 BMW 535xi Monaco Blue Metalic