I’ve found that if you slo-mo to 55 or 60% or faster, you slo-mo is smooth. Any slower than that 50% or less, then you get a flicker, as at 24fps or 30fps you are just duplicating the same frames which gives a stuttery look.
To get true smooth slo-mo at lower percentages 30/40/45%, etc, you would need to shoot with a camera with a “Variable Frame Rate” via capturing a faster frame rate, e.g. 60fps, then slow down to 40%, then you have more frames to work with to create the slo-mo, with a lower rate of duplicating the same frames.
G5 D2, 1.5ram, FCP5, 10.4.2, 23″, Sony GV-D1000 deck, Sony 20″ L2, 250gb FW400 Ext Drives