Trim motor problem

Took my boat to the lake, trim worked perfect until loading. When loading, it was really slow, like the battery was dead. When I hit the switch to raise the motor, it just sounded like it was hooked to a dead battery. I brought it home, the battery has 12.58V on the multimeter, so it is good. I raise the motor up a tad and it goes up very slow, again sounding like a dead battery, let it down, same thing. Then I raise it a little bit and it works perfect, then I lower it and back to slow. What could cause this?? Trim motor going bad? Low fluid?

Motor is a 1990 Merc. 115HP