All shutoff actuators of that vintage operate on vacuum. Whether vacuum is supplied by a mechanical valve at the key switch or an electric switchover valves varies by year and model. Follow the vacuum line, probably brown with a red stripe, from the engine shutoff actuator atop the injection pump. It'll either go through the firewall in which case there's a mechanical valve at the key switch, or it'll go to a small black box with wire connections in which case it's an electric switchover valve.

Sounds like the actuator atop the injection pump is tired if there is some effect but not enough choking to kill the engine. That's my guess, anyway.

Sixto
87 300D