  Spatially inhomogeneous shear flow occurs in entangled polymer solutions,
both as steady state shear banding and transiently after a large step strain or
during start up to a steady uniform shear rate. Steady state shear banding is a
hallmark of models with a non-monotonic constitutive relation between total
shear stress and applied shear rate, but transient banding is sometimes seen in
fluids that do not shear band at steady state. We model this behavior using the
diffusive Rolie-Poly model in a Newtonian solvent, whose constitutive behavior
can be monotonic or non-monotonic depending on the degree of convected
constraint release (CCR). We study monotonic constitutive behaviour. Linear
stability analysis of start up to a sufficiently high shear rate shows that
spatial fluctuations are unstable at early times. There is a strong correlation
between this instability and the negative slope of the (time dependent)
constitutive curve. If the time integral of the most unstable eigenvalue is
sufficiently large then the system exhibits transient shear bands that later
vanish in steady state. We show how perturbations, due to fluctuations or the
inhomogeneous stresses, can trigger this instability. This transient behavior
is similar to recent observations in entangled polymer solutions
