FPS::FPS_control.last_time is my delta time as seconds * 32. I also use it for the entities movement and collision detection, so I only include it here because the sprite jitters on the slope without it I'm not really sure what you mean above.
I'm also planning to add the options to enable Vertical Sync and FPS Lock at 60 FPS in the Options Menu. Does that help?
I don't really want to permanently lock the FPS, though, because I want to always test the performance with a build-n FPS counter.