We study experimentally the flow stability of entangled polymer solutions extruded through glass capillaries. We show that the pipe flow becomes linearly unstable beyond a critical value (Wic 5) of the Weissenberg number, via a supercritical bifurcation which results in a helical distorsion of the extrudate. We find that the amplitude of the undulation vanishes as the aspect ratio L/R of the capillary tends to zero, and saturates for large L/R, indicating that the instability affects the whole pipe flow, rather than the contraction or exit regions. These results, when compared to previous theoretical and experimental works, lead us to argue that the nature of the instability is controlled by the level of shear-thinning of the fluids. In addition, we provide strong hints that the non-linear development of the instabiilty is mitigated, in our system, by the gradual emergence of gross wall slip.