There should be no reason suddenly the idle speed needs adjusted that much, even with the idle speed very low the engine will still run albeit a bit rough.
Sounds like there is air getting into the system since it coincides with deceleration; when the fuel rate slows it gets a big slug of air and dies, the Lucas pump must have 100% air free fuel.
On top of the fuel filter housing there is a banjo, undo it, take the bolt out and look for the tiny hole, clean it out with a strand of wire and make sure you can blow through it.
Then make sure that the pipe it leads to is not damaged or blocked, this is where any air goes that the lift pump brings up, up to the top of the filter housing then back along the return pipe.
Once this has been checked you can check the OUT pipe to the injection pump (the IP is fed from the back where the injection lines come out), so make sure the connection at the filter head is good and tight, and make sure the connection at the back of the pump is good and tight.
There is also a possibility that the governor is sticking; when you take your foot off the throttle particularly in gear as rolling vehicle will keep the engine speed up, the governor will see that the engine is going faster than the throttle is calling for and ALL fuel is shut down to the injectors, it only then throws in more diesel once the speed drops below the idle speed, this system as you can imagine is very sensitive. If the pump is dirty inside or gummed up it can cause sticking and after deceleration it sticks in no fuel position. You could try a heavy duty dose of injector cleaner into an almost empty tank of fuel, or a pint of white spirit (similar stuff and cheaper) to try and clean the pump out.
