My brothers just bought 1999 def 90, which has had a well known company fit a v8 to it. He says sometimes when he drives down the road, it will suddenly run rough like its running short of a few cylinders.
When he stops and turns off the ignition for a few seconds, then starts, its fine for a couple of days and then it does it again.
He has changed the coil, plugs, leads and dizzy cap but it still does it.
I know your all gonna say take it back, but ive said that. Hes a funny [!Expletive Deleted!] when it comes to things like this so i wondered if anyone has any ideas. I think the engine is a 3.9 efi from a rangy. Cheers. :D