I bought my 1998 Patrol 4500GRX at the end of April 2011. It has had a flat spot at about 2500 RPM ever since. A week ago I had a battery package installed (National Luna). Three days ago the altenator gave up the ghost

. I had the altenator replaced (R4400 - Nissan wanted R16800 for a new altenator) with a new one (100Amp), and Voila!! The new altenator seems to work well AND the flat spot is GONE!!! My Patrol now spins smoothly through the whole rev range

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For what it is worth to anybody else.
Jorrie