The instability in stress-controlled cone and plate viscometry is analysed for a bare cone and for a fluoroelastomer-coated cone. We show that, for the bare cone, the instability is viscoelastic in nature, while for a fluoroelastomer-coated cone, the instability is driven by slip. We measure the temperature dependence of the critical stress for instability (slip) on a coated cone and find it to be inversely dependent on temperature. This is expected for the case of debonding, which is known to be an activation process. We also show that the unified slip model successfully predicts the observed temperature dependence quantitatively.