Jackaroo intermittent low revs/ idle fault

I have a 1998 3 Litre Diesel Jackaroo that has developed an intermittent fault.
At idle or at low revs the engine occasionally coughs/lacks powers. At Idle it will drop from approx 800 revs to 400 revs for few seconds. This problem only begins after the motor has warmed up or in cold conditions when the engine heat up switch turns off. It is particularly noticeable whilst travelling downhill in 4th or 5th gear at 1500 revs or lower. To date I have replaced the fuel filter, checked fuel for contamination, had the throttle body de-carboned, the EGR value removed, cleaned and checked (previous EGR valve has already been replaced), also had injector wiring checked.
Any suggestions on what might be causing this problem?