If your car were hydrolocked, it would be just that.....locked up. When my car did that, it was so stuck that the starter shattered when it tried to turn an unturnable engine.
Does your engine still turn over and start? It just turns slowly???? If so, the first thing I would suspect is the battery and its connections. Hot engines/cables/batteries create conditions that expose those issues.
If those are all topnotch, my next step would be to evaluate the starter, as it may be in the beginning stages of its DeathSpiral.
I doubt that the uphill part of your diagnosis has any bearing on your problem as current doesn't know whether it'* flowing uphill or down.