  We study the strain response to steady imposed stress in a spatially
homogeneous, scalar model for shear thickening, in which the local rate of
yielding \Gamma(l) of mesoscopic `elastic elements' is not monotonic in the
local strain l. Despite this, the macroscopic, steady-state flow curve (stress
vs. strain rate) is monotonic. However, for a broad class of \Gamma(l), the
response to steady stress is not in fact steady flow, but spontaneous
oscillation. We discuss this finding in relation to other theoretical and
experimental flow instabilities. Within the parameter ranges we studied, the
model does not exhibit rheo-chaos.
