Hello P84R,
The first thing I would do is to take off the water gauge fittings and check that their passageways and the areas around inside the boiler are free of obstructions that might give a false water gauge reading.(Check also that the bottom fitting and bush does not protrude too far into the water space).
You might also be getting priming when the loco is being worked hard, as others have suggested. Also you do not state whether the safety valves are lifting as you go up your incline. You can get a localised pressure drop in a boiler that can promote priming, as opposed to priming being caused by dirty water or water with some oil in it.
Water carry over to the cylinders is quite a serious problem and can cause damage to the cylinders and motion, if not increased wear, and ought to be properly investigated.
If there is a water carry over, you need to shut the regulator and open the cylinder drain cocks.
Do a search online for LNER pacific 'Blue Peter' and the disaster at Durham.
Cheers,
Julian