  We revisit the H\'ebraud-Lequeux (HL) model for the rheology of jammed
materials and argue that a possibly important time scale is missing from HL's
initial specification. We show that our generalization of the HL model
undergoes interesting oscillating instabilities for a wide range of parameters,
which lead to intermittent, stick-slip flows under constant shear rate. The
instability we find is akin to the synchronization transition of coupled
elements that arises in many different contexts (neurons, fireflies, financial
bankruptcies, etc.). We hope that our scenario could shed light on the commonly
observed intermittent, serrated flows of glassy materials under shear.
