Air in the fuel system. Install clear pipe from Injection pump to filter and observe for air in the fuel. Alternatively connect pipe from pump inlet to container of fuel (by-passing the rest of the system). If the engine
runs OK, then investigate fuel supply from filter to tank.
Blocked return pipe
Blocked inlet or return banjo bolts.
adjustment of idle speed screw, also check adjustment of the anti-stall screw (If fitted)