cruising 60 mph for about 130 miles, oil pressure at a steady 51 psi, temp
at about 180 deg, all of a sudden the oil idiot light comes on.  slow down,
put in clutch to downshift, light goes off.  let clutch out in third, light
comes back on.  so: trans engaged=light, trans disengaged=no light.  in
addition, the light seems to flicker with the speed of rotation of the
engine.  come to stop.  light is off.  no sign of anything visibly wrong. 
vdo guages read as always.  start up again, light comes on briefly, then
goes away for the last 40 miles of the trip.  any ideas?

thanks, matthew