Give cinema tools a try. right click footage from Finder, open with cinema tools, choose conform.
Last week I was given some footage that was shot in 24P,, just dropped in timeline and rendered it looked awful.. Had little time before the piece had to go to air so I ran it through Cinema Tools.. Had never used Cinema Tools before,, but in minutes conformed the footage to 30FPS. The footage played faster in the FCP timeline,, but was totally smooth, just adjusted the playback speed and it was perfect.
I’m sure there probably is a better/propper way to deal with this,,, but in a pinch it saved my butt and playback was totally smooth.
Colin McQuillan
Vancouver BC