Playback only stutters in preview screen, not full screen.

Brand new iMac 5k, maxed processor and graphics driver, and loaded up with 32gb of ram. Files live on internal 1tb drive. Originally 1080p 30fps MP4 with h.264 codec. Have created proxy, have optimized, have used compressor to switch to prores format. The files play, freezes, then plays again and freezes. Occasionally I get a dropped frame alert when playing on timeline or when previewing original file. Files do not stutter when doing full screen playback! I have turned on and off background rendering also. This is my first foray into FCPX so any help is appreciated. Timeline is set to match the frame rate of clips when I load it in. Beta testing the new Yosemite, could that be it?