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It could be a number of things...First change the spark plugs,check hoses not just intercooler but all boost hoses.If it is none of them it could get expensive.Maybe the gaskets on the inlet manifold,top one is easy to do but the bottom is a nightmare.If you don`t know what you are doing though it is best to get it to a specialist...North try middlehurst or rod bell,south GTart or abbey.
Look on www.gtr.co.uk might be more help.
If you disconnect the battery make sure the front wheels are dead straight when you reconnect or you will confuse the 4 wheel steer and be crabbing all over the place...