200tdi not ticking over (cutting out/stalling) help. new member

Can anyone help? My 1990 200tdi rangie will not tickover as it should. she'll start really well...tickover as normal for as long as you wish... increase the revs a bit (1500 or just over)... then let her go back down to tickover... very lumpy, close to stalling stalling...if you increase the revs... to the floor...she'll take ages to respond...and eventually revs up all the way....then gently releasing the throttle, only to find the annoying lumpy stalling fault again.

I've changed the lift pump...no joy. Replaced the fuel sender...no change there. I'll be changing the fuel cut off valve this week, unless anyone has a good idea. The water catchment filter on the offside rear chassis looks old, but the plastic screw is not siezed. ...

please advise