Instead of increasing the width of M1 (it should be a long transistor anyway, s. below), try reducing the width of M3.
Oscillation originates from not enough gain in the (positive) feedback loop during the switch-off period. Also try increasing the width of M2 - so increasing this gain. M2 is totally off after start, so no current through it, then. Only M1 - M3 leads current after startUp, which is determined by the W/L ratio of M1. That's why M1 usually is a very long transistor (W/L « 1).