  We investigate shear thickening and jamming within the framework of a family
of spatially homogeneous, scalar rheological models. These are based on the
`soft glassy rheology' model of Sollich et al. [Phys. Rev. Lett. 78, 2020
(1997)], but with an effective temperature x that is a decreasing function of
either the global stress \sigma or the local strain l. For appropiate
x=x(\sigma), it is shown that the flow curves include a region of negative
slope, around which the stress exhibits hysteresis under a cyclically varying
imposed strain rate \gd. A subclass of these x(\sigma) have flow curves that
touch the \gd=0 axis for a finite range of stresses; imposing a stress from
this range {\em jams} the system, in the sense that the strain \gamma creeps
only logarithmically with time t, \gamma(t)\sim\ln t. These same systems may
produce a finite asymptotic yield stress under an imposed strain, in a manner
that depends on the entire stress history of the sample, a phenomenon we refer
to as history--dependent jamming. In contrast, when x=x(l) the flow curves are
always monotonic, but we show that some x(l) generate an oscillatory strain
response for a range of steady imposed stresses. Similar spontaneous
oscillations are observed in a simplified model with fewer degrees of freedom.
We discuss this result in relation to the temporal instabilities observed in
rheological experiments and stick--slip behaviour found in other contexts, and
comment on the possible relationship with `delay differential equations' that
are known to produce oscillations and chaos.
