  We investigate the conditions leading to large drag force fluctuations in
granular materials. The study is based on a set of experimental drag tests,
which involve pulling a plate vertically through a cohesionless granular
material. In agreement with previous observations, drag force exhibits
significant and sudden drops -up to 60%- when the plate is pulled out at low
velocities. We further find that this instability vanishes at higher pullout
velocities and near the surface. We empirically characterise the frequency and
amplitude of these fluctuations and find that these properties are not
consistent with a classical stick-slip dynamics. We therefore propose an
alternative physical mechanism that can explain these force fluctuations.
