How full is the fuel tank when the problem occurs? Wonder if there could be an issue with the pickup that shows up at those higher angles. Fuel, air, compression, you need all three, my guess is a fueling issue under heavy load... Next time you have the problem:
1. Check how much fuel is in the tank
2. Go back down to the bottom, and try reversing back up the hill. This could help determine if the operating angle has anything to do with it.
3. Go back to the bottom, and go up forward again and see if the problem happens at the exact same spot.
4. Go back to the bottom, fill the fuel tank all the way, and try again and see what happens.
You might try each step a few different times. If you can determine the problem can be replicated consistently, under the same conditions then you can start to eliminate potential causes.