Vehicle: 96 XJ

Problem: Runs rough at all RPM, and frequently shuts down upon deceleration. Idles at a normal RPM despite roughness. When it hits around 3.5k RPM the roughness becomes very pronounced and choppy, to the extent that 3.5k usually cannot be exceeded. When the engine is between 2.5k-3.5k RPM and the accelerator is released (to brake, for example), the RPM often drops straight to 0 and the engine shuts down.

Any ideas on what the issue could be? My first guess is that there could be a fuel delivery issue, but at last check the pressure was fine. Fuel quality issue?