Unfortunately the zd30 is one of the worst diesels Nissan ever made, a far cry from the earlier TD series diesels. I deliberately bought an earlier qd32 powered terrano not a zd30 for that reason.
The zd30 is known for melting piston 3 and blowing turbos. It's regarded as a matter of when, not if, it will give you major grief, usually around 170,000 kms or so. Unfortunately this is not a cheap fix and I have seen several NZ Terranos with blown zd30 diesels that have been retrofitted with a qd32 or td27 diesel.
If your boost pressure is fluctuating I'd be worried about the turbo. Could be worth taking the intake side to pieces and checking nothings loose. Does it blow any smoke?
Also try offroadexpress.kiwi for more info.