just to be clear.
it lifts fine cold.. but won't lift hot?
the 'drifting down' is the 'when it gets hot part..? and not because the 3pt control is falling is it.?
if the friction disc is worn, and lets the 3pt handle fall.. the lift will of course drop.
if it's the war / heat issue..then yeah... pump work time.. or replacement as you are doing.. etc..
however for giggles.. you could pressure test it when hot.
after getting hot, chain upper lift arms to axles, plumb in a 0-3000 psi gauge to the plug in the 3x5 cover under the seat.. or even intot he head of the pump.. and then exercise the lift and see what pressure she makes.. yours likely won't hit relief pressure as it is probably bypassing..
i'd also look into the sump and see if there is excessive leakage from the top side..