  The competition between the drive and stabilization of plasma
microinstabilities by sheared flow is investigated, focusing on the ion
temperature gradient mode. Using a twisting mode representation in sheared slab
geometry, the characteristic equations have been formulated for a dissipative
fluid model, developed rigorously from the gyrokinetic equation. They clearly
show that perpendicular flow shear convects perturbations along the field at a
speed we denote by $Mc_s$ (where $c_s$ is the sound speed), whilst parallel
flow shear enters as an instability driving term analogous to the usual
temperature and density gradient effects. For sufficiently strong perpendicular
flow shear, $M >1$, the propagation of the system characteristics is
unidirectional and no unstable eigenmodes may form. Perturbations are swept
along the field, to be ultimately dissipated as they are sheared ever more
strongly. Numerical studies of the equations also reveal the existence of
stable regions when $M < 1$, where the driving terms conflict. However, in both
cases transitory perturbations exist, which could attain substantial amplitudes
before decaying. Indeed, for $M \gg 1$, they are shown to exponentiate
$\sqrt{M}$ times. This may provide a subcritical route to turbulence in
tokamaks.
