another thing to consider is..... Is the battery staying charged? this type of Clutch/PTO pull quite a bit of power from the battery, if the battery is weakened or not charging enough to keep up with the power being drawn from it, can cause the clutch to kick off, give the battery a good charge then run the mower, if the clutch stay engaged and doesn't kick off, then I would next check into the charging system, or possibly a new Battery,