you have a vacuum leak somewhere. those old diesels use a vacuum shut off switch to turn the engine off. check all the plastic vacuum lines under the hood and also inside the dash going to the ignition switch.
The SOV takes the vacuum of the brake booster tube in front of the inner fender. Follow the connection and small tubing going via firewall for disconnection. Than follow the tubing going to SOV from the firewall. If that looks good -check the vacuum switch at the ignition.
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