Complex sentence: While long-lived this orbit was unstable on a time-scale of 600 Myr, eventually perturbations from the other inner planets drove Planet V onto a high-eccentricity orbit which crossed into the inner asteroid belt.
Split into simpler sentences: While long-lived this orbit was unstable on a time-scale of 600 Myr. Eventually perturbations from the other inner planets drove Planet V onto a high-eccentricity orbit which crossed into the inner asteroid belt.