Just brainstorming possibilities . . . someone may have monkeyed with the slide spring, as in clipping coils.
Clipping coils changes the initial spring pre-load, but . . . counter-intuitively, clipping coils makes the spring STIFFER (i.e., higher rate, force vs. displacement). Thus, at the higher slide openings (higher throttle positions), the increased spring rate of the compressed spring may resist slide elevation, compared with a stock slide spring configuration.
“You better put down that gun. You got two ways to go, put it down or use it. Even if you tie me, you’re gonna be dead.” "John Russell" (Paul Newman), Hombre