Hardware based slow motion recording on this phone seems to cover 1080p at 240 fps at the most.
For native 960 fps, the camera sensor apparently needs an embedded DRAM to help process all the data, which actually most phones don't have.
Those that don't - and still offer a 960 fps mode - rely on a software-based approach, with software processing and interpolation to basically add duplicate frames and artificially create a 960 fps clip.
And about flickering - I've only seen it occur when trying to record a scene involving LED lights. Which is perfectly normal, if the LED lights are powered by alternating current. They basically turn on and off about 100 to 120 times a second (depending if electricity runs at 50hz or 60hz). Your eyes don't pick that up, but the camera will. That is, if you set it to high frame rates - to eliminate flicker, try 30fps.