The clutch engages fine when mower is first started up. After running for an hour or so and warming up, clutch will not re-engage if you release it. 1. Local maintenance shop says almost surely the clutch is going bad and should be replaced. Does that sound reasonable? (It does to me, but I've never had one go out before and am not sure why it won't re-engage because of being warmed up) 2. Is that an easy DIY job or a little difficult for someone with very average mechanical skills and facilities?