200tdi stalling issue

Son took his 200tdi out yesterday for a couple of hours and it drove as per normal. Today on the way to work it drove fine but when coming of the throttle it seems like it would momentarily stall then pick up idle again. Other times it will be fine and sometimes it will just cut out and not idle at all. Starts fine, switches off fine, drives fine, just doesn't like to idle especially after coming off the throttle.
I have put a new connector on the stop solenoid wire, bypass the vehicle wiring for stop solenoid, no difference.
Lift pump isn't leaking and gives a good squirt of fuel when operated by hand.
That's as far as I have got, what else should I look at ?

How much slack in the throttle cable?
Could the idle screw moved?
Crap diesel,when was the last fill up?
Crap in filter(could be water related as cold mornings causing dew in tank)

Not sure when he last filled up, it hasn't been started for a few weeks. Think there is a spare filter in the garage, might fit that tomorrow if I get chance.
Throttle cable is fine, don't think the idle stop has moved, besides sometimes it will idle just fine, mostly not though.
Thought my luck was in with the solenoid connector as it looked a bit dodgy. When it stops it's quite sudden as if it's turned off, other times it runs lumpy as if fuel restriction, sometimes it fine, annoying.

Leak off pipes, worth changing?

Anaconda said:

Leak off pipes, worth changing?

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Normally they allow fuel/air back to the tank when turned offf. With the engine running they should be fine.
remove the cotton outer on the spill pipes & look for cracks.

Could still be lift pump issues, or the pump is drawing air from somewhere?