If you have it use Time Warp in After Effects instead.
There is an anti flicker filter in the Field Options or add a very slight blur.
or of its interlaced material you can deinterlace the clip.
Change speed in whole numbers like 50%, 25%.
I'm not a fan of Premiere Pro's current slow motion capabilities. I agree with Richard, doing this in After Effects with the Timewarp effect you'll get significantly better results.