Thank you for being so detailed in the question.
I suspect the belt could be slipping. First, you should clean the debris out from the clutch mechanism. Grass can get packed up and cause the belt to slip. Please use the image below that shows the area of the tractor that should be cleaned. I recommend using a leaf blower or compressed air to remove the grass and dirt. After the area is cleaned, test the tractors operation. If it still slips, hook your foot under the brake pedal and pull it back when going up the hill. This will tighten the belt. If the tractor works like it should I recommend replacing the belt.
Here is a link to order any parts you need. Sears Parts Direct
If you do not feel confident repairing this problem yourself, then you can have it repaired at your home by a Sears technician. Here is a link for the website: Sears Home Services
If you need further assistance, please reply below with more details.
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