Fluctuating idle

[EVOIV]

Hello,

I have a small problem with the idling on my car. It's a 1997 EVO4. Sometimes, it idles fine at 1000rpm (yes, that's high!!) and the car feels nice and smooth...but then after a few seconds it slowly drops to 800rpm and the idling starts to get a bit lumpy. Then it shoots up to 1000rpm, then a few seconds later drops to 800rpm...etc. Does anyone know why this happens? It sounds like it could be due to the ECU? Otherwise, what else would make it fluctuate as such?

History: Bought car standard, idled fine at around 800rpm is I recall correctly. Then did mods to it which included: HKS turbo, manifold and wastegate, MoTeC ECU (stand alone, factory ECU was removed), 044 Fuel Pump, max boost was set to 1.2bar and the idle was set to around 1000rpm. Now, the car has been returned to almost stock: Stock turbo and manifold, stock ECU, stock intank fuel pump. And now, I get the idling problem as mentioned above. It's no biggy, cos the car still idles fine and doesn't stall...just that it's a little annoying problem

Could it be due to a vacuum/boost leak? If so, where should I check? And how can this be done?

Cheers for any help provided...