When you said you tried each roller by hand did you pull the chain off of the crank and spin each gearbox? Or pull the chain off between the gearbox and each roller and spin roller? We had a metering gearbox seize up on us this year and it manifested itself by stopping all the other rollers and slipping the master clutch while seeding, you would not find the seized metering gearbox if you checked each individual roller. I had a buddy of mine with an '09 cart and he replaced all three gearboxes last year in one season.