I've never heard of a 35-80mm G lens, but an AF-D 35-80mm F4-5.6 exists.
I suppose you are setting the aperture ring to the minimum aperture, typically F22 and locking the ring with the switch in order to work in P or S modes. Do that and set the camera to P. Does it happen again?
If so, then you need to send the camera to be repaired. In the UK, I'll recommend Fixation or Aztech Services.
They'll do a CLA and you'll get an F4 as new as the day it was born.