We have observed abrupt, reversible resistance changes during electromigration (EM) testing of 0.16μm thick, 100μm long damascene Cu conductor lines with TaN liners and W vias. Lines of 0.2μm, 0.8μm and 2μm widths were tested and compared. The tests were conducted at temperatures of 300C – 350C and with current densities of 0.6 × 106 A/cm2 to 1.6 × 106A/cm2. In most cases an incubation period with negligible resistance increase is followed by a period of continuous, gradual resistance increase, attributed to formation and growth of voids in the conductor line. With further EM, the resistance curves show spike like features, with a sudden resistance increase, followed by a resistance decrease, often to values close to those before the start of EM. In other cases, no resistance decrease occurs, and the line fails. We present resistance data, microstructural observations and thermal calculations that suggest that the resistance decrease results from sudden, local Joule heating melting of conductor line segments and voids being partially filled by the back-flowing liquid Cu, which then solidifies. In some cases line failure results from liquid Cu erupting through the top surface passivation layer, rather than flowing back to fill voids in the line.