When mower has been running for aprox 1hr.+, when blades are disengaged by switch, clutch will not re-engage. Seems that the circuit is good, switch tests good, but clutch only begins to 'chatter'. Eventually after cool down or push/pull switch, many times, clutch engages. I think that the clutch is beginning to go. Mower has about 500 hrs. Any suggestions would be appreciated.