Stretching footage will cause frame blending or frame doubling. Your first step should always be to change the frame rate using File>Interpret Footage>Main. Stretching footage is an inaccurate way to create slow motion. As Byron said, the footage layer you warp stabilize must match the frame rate of the comp.
Then you can create a new comp from your footage and run warp stabilizer. If you want to work with 24fps comps then interpret the footage as 24 fps. If your original footage was 120 fps and you want it to playback so that one second of real time is 4 seconds of screen time then interpret the footage as 30fps. If you want to change the frame rate change the interpretation of the footage.
