I have a 98 k3500 with an oscillating fuel gauge.  For example when the
indicator shows 3/4 of a tank the needle will oscillate from about 5/8 of a
tank to 7/8 of a tank.  I am guessing at the rate of oscillation but would
say it oscillates at about 15hz.  I have owned the vehicle for about 3 weeks
and it was doing it when I bought it, so I am not sure when it started doing
it.  So is it something in the instrument cluster or in the fuel tank?  All
of the other gauges work well.

Thanks