One of the symptoms of a worn hydraulic pump is it seems to work fine when the oil is cold and thicker, but as the fluid warms up and thins a little, it starts to leak inside the pump, and things slow down. This applies to other components in the lift circuit as well, like port relief cartrides or the main relief, so it is not necessarily the pump. A direct pump test with a flow meter can eliminate the pump.
Depending on the serial number, the pump may be easy to pull and open up and see it it's toast, if you don't have access to a flowmeter. An 843 high-flow with external servo steering is not such a piece of cake to remove the pump.
Just a thought.
I'm not quite following about the tilt falling while raising? It may be that you have more than one problem at the same time. Lots of 843's had a bucket-levelling valve, and these have cartridges in them that can wear also and work fine when cold but screw up when warm.