This evening I connected the J-Head MK Vb. It involved some fiddling with the tiny connectors to make the double connection at the Sanguinololu end (see here for the wiring diagram), but I managed. The temperature is showing in Pronterface for Mac and the hot end gets really hot. So, one more thing to tick off on the todo list and sofar for the good news.
As for the motor that goes crazy (x-motor position; first connector from the left) after starting a 'dummy print', no progress is made so far. Hoping to isolate a faulty wiring or Pololu, I first switched the X and Y motors positions with no result. I also switched the Pololus without a positive result. The motor connected on the leftmost Pololu still acts bizarre.
The only thing left for now is to go back to the original firmware (standard Marlin) that was preloaded when I received the Sanguinololu and see what happens.
If the 'crazy motors' insists on spinning until I reset the printer in Pronterface, the board must be faulty; if not there might be an issue with the 'Rostock modified Marlin'. Hope to have some good news soon.