On a 1997 Isuzu Trooper 3.1 diesel turbo with an electronic pump and the speed at idle is not stable it varies from 700 to 1500 rpm and the car is shaking what could be wrong?

As a blind guess, the ECM is basing the Air/fuel mixture with incorrect data from a bad sensor or vacuum leak. Check the Idle air control actuator, It's responsible for allowing more or less air to bypass the throttle plate to stabilize the idle. It gets commands from the ECU based on information from various engine sensors.