G'day herbie , mate I recently had a similar problem symptoms like a NPS but it was still doing it after unplugged , it was only down on power and surging a bit every now and then so I new there was really nothing to serious going on and it would over boost , one time up to 22psi, so starting checking sensors had a spare map/boost sensor so I put that on thought I had it , but started playing up again , so kept checking , I put a uni filter in a few weeks back and it occurred to me that it started playing up then , I think I've got it fixed now ( I hope )turns out the oil (and there was a lot in the bottom of the air filter box) was fouling the map boost sensor , Its been OK for a while . So long story short maybe check , clean boost sensor .