Object movement choppy when frame rate is capped

I've been having some trouble with choppy movement of objects in a game.

If the frame rate isn't capped, the game runs smooth.

If I cap the frame rate at say, 60 fps, the movement is choppy.

An object's movement looks like this...

m_distance += m_speed * GetFrameTime()

Where speed is in pixels per second, and GetFrameTime() returns the time elapsed since the last frame. Note that objects only move in x and y directions.

I'm using SFML, with the functions SetFramerateLimit(60) and UseVerticalSync(true). While trying to address the choppy movement issue, I read an article from an XNA developer about how they update once, render once, and then if there is time left in the game loop < the frame rate (i.开发者_如何学JAVAe. 1/60) then they'll sleep for the rest of that period. I believe this is what SFML does, because I disabled those functions and wrote in that behavior and I got the same symptoms.

Now I'm not saying the above approach is a bad thing. In fact, I like that the application isn't doing needless work (there is a noticeable difference in resource usage when running the application with a set frame limit and without one). However, I really don't know why the movement is choppy. I've read the gafferongames articles but they don't seem to apply here.

Any assistance is appreciated, thanks.

Edit: I've identified the the issue, now just not sure how to solve it.

On the frames where the object "jumps", the m_distance is significantly greater than distances in other frames. On closer inspection, the GetFrameTime() is also significantly greater than previous times between frames (significant is like 2-3 ms difference).

Edit2: I believe I've come up with a solution thanks to http://gafferongames.com/game-physics/fix-your-timestep/ . Set a fixed delta time instead of using GetFrameTime.

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The only reason to ever use:

m_distance += m_speed * GetFrameTime();

is if you're trying to get frame-independent movement. In this case, there's no reason to cap your FPS.

When you have a FPS cap such as what SFML implements, I recommend keeping movement constant per frame. That is to say, something like this:

m_distance += m_speed;