What do you mean by crazy and choppy? Do you mean the speed transition between normal speed and the slow motion is abrupt or do you mean the portion that’s in slow motion is choppy?
If the latter then make sure the clip is rendered to display normal playback without dropped frames. If you want a smooth transition between when the speed of the video goes from normal speed to slow motion (time remapping with speed ramp) then use the range select tool, select the range you’d like to change the speed of the clip on, then press shift-n to create a retime segment for the selection. Then you can drag the handles to retime the selection and you’ll get a smooth speed transition where the speed ramp occurs. If you’re doing something like using the blade tool to “cut” the part you want to retime and then doing the retime on the “cut” clip then you’ll get an abrupt look on your video where the normal speed clip suddenly starts playing the slow motion clip where the “cut” is.
Also try enabling optical flow as the retime option. Hope that helps!