"I know that the throttle valve controls the fuel delivery up to 1500 rpm."

This statement doesn't jibe with my understanding of D-Jetronic, which I have researched in great detail.

My Cosworth Vega has a Bendix electronic fuel injection system. Bendix was not able to sell their system to the Detroit OEMs in the sixties, so they licensed the patent rights to Bosch and Bosch produced the D-Jetronic, which is generically a speed density system with analog electronic control. The first prototype Bendix speed density EFI system was built and installed on a Buick V-8 circa 1953. It used a vacuum tube control unit about the size of a small AM radio of the era.

My CV has both Bosch and Bendix components.

There are two critical sensors that this system MUST have to function. The MAP sensor and the speed sensor. I can disconnect the coolant and air temperature sensors and the throttle position sensor and the engine will still start and run.

So check both your speed sensor and MAP sensor using whatever test specifications for your particular components that you have or can find, and be sure to carefully inspect the connectors to these components. It's possible that engine vibration at 1500 is causing an open.

On any old car, connectors are the most common source or electrical anomalies.

Duke