Are your Slo-Mo effects stuttery? Sometimes it is unavoidable when using the Traditional (Non-TimeWarp) Motion Effects. But, here is the best way to make sure you have done all you can to make your slo-mo's and fast-mo's as smooth as possible when creating Traditional (Non-TimeWarp) Motion Effects. Essentially, its a combination of math and the Two-Field Rendering Type you select.

 
The Math:
The key is to make your frame rates EVEN multiples and divisors of the frame rate you are using (which in our case here in the US is 30fps, and for you PAL folks its 25fps). So, for slo-mo, make them EVEN divisors: 15, 10, 5, and 3 frames per second. For fast-mo, make them EVEN multiples: 60, 90, 120, 150, etc.

 
You can still use the FIT TO FILL Check-Box, but just use it as a way to determine the approximate fps. Then, when possible, defer to the closest EVEN frame rate. This does not mean you can't use other frame rates or fit to fill. This is just the way to make them as smooth as possible.
  
The Two-Field Rendering Type:
The "Duplicated" field rendering choice is the least desirable. If there is movement in the shot (which means there is "inter-field motion"), then generally "Interpolated" is the best choice. If you are on a system with VTR Style rendering, you can try that too and see if it gives you a better result than interpolated. Realize that these choices for rendering will only be noticeable if you are at high resolution (meaning with two fields).