mower starts and runs fine on level ground. Going uphill will stall in a few seconds.
going left to right on hill runs fine. Going right to left on hill will stall in a few seconds.
It sounds like the fuel level in the carburetor bowl may be too low or you may have water in the fuel.. Change the fuel in the tank. Then take the bowl, float and float valve out. Clean those and stick it back together. That may take care of it.