Yes I have heard of a bad spot. I've had some cheap starters that would not do anything, even with a hot battery. I didn't hit the starter with a hammer, although that may have worked, I took the plain end of my pretty nickel plated lug wrench and shorted momentarily between the hot cable at the solenoid and the large cable going to the starter motor itself. CAUTION, when I did this, it was after checking about five times to ensure that the transmission was in neutral. After giving it a quick spin like this, I was almost always able to start it with the key normally.
I'm not sure if this situation is a bad spot on the commutator, or a flaky solenoid.
After rereading the original post, you said that there was 12 volts at the battery at idle. If this is the case, the alternator is not putting out. You should measure about 12 volts with the engine stopped, and about 13.5 to 15 volts with the engine running. If you have 12 volts with engine running, you either have a bad alternator, or a problem with the associated wiring. I had to resolder the heavy wire in the alternator cable a few years ago. It will show up as a bad alternator unless your testing thoroughly with your multimeter.
Also, don't rely on the alternator light on the instrument panel. Benzmac said that the light circuit passes through the brushes. Since my alternator (battery symbol) light on the instrument cluster has rarely been of any use when I've gone through these problems, what he says makes good sense (as does most of what he says).
Sounds like you need to check the battery voltage again. If it's 12 volts with engine running, check your alternator wiring, if it's okay, replace the alternator or the brush pack.
'84 Euro 240D, 516K miles
'88 300E 5 Speed
'81 300D Daughter's Car
Over 800,000 miles in