Hi, first make sure the grey card is in even light - with no shadows or highlights or overly reflecting areas. Make sure you are completely filling the frame with the grey card - and I mean completely - anything else can really mess this set up.
Set the camera to manual mode, select 1/60sec and using the meter display adjust the aperture until the indicator indicates a correct exposure.
Before you do that make sure that any exposure compensation is cancelled - I had a module two last week where the student was unaware they had the exposure compensation set to plus 1 stop, so that is one place to look first if you are getting over exposed pictures. Seeing as how you are adjusting that at times to get a correct exposure it seems unlikely (but do make sure you have cancelled it after using it.)
If they card is measuring the same 158 in all channels then it's not a white balance problem. The ISO shouldn't have any effect on whether or not the camera gives a correct exposure - it will only change the shutter or aperture setting to suit the sensitivity selected.
If you are doing everything above and your card is still coming out consistently too light with no other camera control set to over expose then the camera is at fault and you should get Nikon to look at its calibration.