This paper explores rheological characteristics of and molecular mechanism for a superfluid-like stick-slip transition occurring under controlled pressure in capillary flow of a series of highly entangled linear polyethylene (PE) melts and establishes its connection with the spurt flow phenomenon. The transition is signified by a large discontinuity in the flow rate at a critical stress, resulting in a double value within the flow curve. The magnitude of the transition can be quantified in terms of an extrapolation length, b. In particular, the superfluid-like flow transition occurs throughout a range of temperatures from T = 180 to 260 degrees C as long as a critical stress, sigma(c), is exceeded. It is found that sigma(c) increases linearly with T, and b(c) at the transition remains around 1.7 mm at all the temperatures for the PE (MH20) of weight-average molecular weight M(W) = 316 600. Thus the observed remarkably large interfacial slip is believed to be due to complete disentanglement of the adsorbed chains from free chains at the melt/wall interface at and beyond the transition. The amount of wall slip, as described by b, diminishes quickly with decreasing M,, in qualitative agreement with a simple scaling relation for noninteracting interfaces. The flow transition depends on the surface condition of the die wall and occurs at a considerably lower critical stress when the wall is treated by depositing a fluorocarbon elastomer to weaken the PE adsorption. Application of both controlled-pressure and controlled-piston speed conditions demonstrates that spurt flow instability originates from indeterminacy of the hydrodynamic boundary condition at the PE/die wall interfaces.