Hi Mike,
I presume a full diagnostic systems scan has been carried out with no faults logged?
Faults that may cause your fault;
1.Throttle Valve flap contaminated, causes sticking flap when hot.
2.Throttle Valve incorrectly base set.
3.EGR Valve incorrectly base set, sticking slightly open - possibly due to cat substrate disintegrating & finding it's way into the EGR valve.
4.Alternator load sensing wires broken at 2 pin plug at the front of the gearbox - load signal lost - affects idle behaviour in the bizarrest of ways, cutting out, hunting etc....

Jon.