Power trim losing pressure.
I thought that I had blown a seal on my trim unit, as it was leaking fluid. Turns out that that seal or cap on the top of the ram was loone. But if you are slowly slipping down, I would suspect low fluid. How long does it take to slip down?
Sounds like you may have a seal leaking internally if you are losing pressure without leaking any fluid.
First guess was the fluid is low.. I think you have a seal going bad..However what doesn't make sense is that the fluid is still full. I have had it happen on bigger outboards and the fluid ids low and I see just a little wet spot.