Symptoms of bad pump...need urgent advice??

[wasteland_TD]

Hi everybody...
I think that my my injection pump is not working properly.
The motor is 1.9TD AAZ and the pump is with LDA device from 1.6TD.I bought the car with this configuration mod.
The sypmtoms:
When the car is cold or worm the idling RPM are perfect, but if I drive the car on highway with 140-150km/h for about 30 minutes the idling RPM drops down - around 600-650rpm(i have tachometer) and because of that there is unpleasant vibrations.
After 5-6 days or 2 weeks, it depends, the idle RPMs gradually increases by itselfs.
The same thing is when drive the car in town for about 5-6 hours almost continuously - when it gets hot - the rpms drops  :evil:  :evil:
and when that happends the car runs notable poor.There is no power especially in lower RPMs.
There is something else....
If I go downhill, take off the foot from the gas pedal and the RPMs are between 2000-2300 there is notable interruptions - like there is no fuel.So I must go under or over there RPMs to drive normal.If I press the gas pedal even slightly these interruptions disappears.
I think this is injection pump problem, because anything else by the engine is almost perfect....
I have changed the piston rings, the lifters, head gasket, also changed the nozzles with GTD nozzles, intercooler too.The problem is still there.
Injections pump timing is set to 0.83.
I have never add more fuel from the pump.
However the engine start very very well, even when the Temp. is -20 degrees.
The fuel consumption is great 5.5-6Litres/100km
So what is your opinion - is this pump problem?If it is , how bad is?Can that be repaired?
Because I have found are used pump for about 200US$ or it is cheaper to repair my pump.
The good in my pump is the LDA device from 1.6TD   :roll:
I am wondering what is the best solution in this situation....

10x for any help

[QuickTD]

Injections pump timing is set to 0.83.

Fix this first, the timing should be set to 0.87mm stock. If you want the best power and economy set it to 0.95 or even a bit more. At 0.83 you will get misfiring and at light throttle. You should also disable the "load dependant timing" by connecting the timing solenoid to the fuel cutoff solenoid.

 Some idle drop is normal due to the density of the fuel dropping as it heats up, 50-100rpm driop is common. If yours is dropping more than this you may need to adjust the "residual pressure" on the injection pump to get the idle governor to function correctly. Fully warm the engine (until the idle drops), then back out the screw that the throttle lever rests on at idle, keep turning it out until the idle stops decreasing or hits about 800rpm. Tighten the locknut. Now adjust your idle back up to 850-900rpm using the normal idle screw on the rear of the pump.