I'm not familiar with this particular car, but doesn't it have the K Jetronic as others of that era? If it does, part of that system is a knock sensor that prevents this from happening. If it is a K jetronic, there are two modules, a fuel injection control module, and an igniton control module. The knock sensor connects to the ignition control module and one of these units or the associated wiring is at fault.
Hope this is found to be helpful,