low tickover and cutting out sometimes.

Hi , ive got a 1996 4.0 litre p38. now sometimes when cold it will start perfect but when you take foot of gas pedal the revs drop down to nothing and stops. it does this sometimes at junctions then displays gearbox fault.
after a few mins of running it will work fine and ticks over perfect.
ive took it to my local landy shop and had the diagnostics dond and all ok, but they said it ticks over quite low around 550 but even then it is smooth.
any ideas please

Does anyone have any ideas. ive checked the stepper motor and thats fine. all was clear on diagnostics too so not afault on switch or something.
just very strange that it ticks over very smooth at around 550 then sometimes wont tickover at all. no sooner has the fault come it goes as quick and shes back to ticking over smooth ahain.
any help would be great

There is obviously something wrong,it should idle all day with no fault,did your indy monitor fuel trims and oxy sensor activity ? Or did they just look at fault codes ? The target idle speed is set by the ecu,and will be achieved and held with constant adjustments of fuel trims,ignition advance and iacv position.For whatever reason one of these is not happening,so the indy needs to focus on them,viewing live data or scoping sensors etc.

The idle can be set by testbook but as pointed out there are other issues to consider.
Has it been serviced recently?

Yes its had full service. headgaskets done as well.its only when you first start her up that it drops to low , after about five mins she runs perfect and is so smooth that sometime you think its stopped but it hasnt , its just very quiet and running well.
very odd