Choppy playback in final cut pro x using go pro footage

Hi guys, I know this question's already been asked but i cant find any answers to my particular circumstances. I'm running FCPX on an up to date 2013 iMac with a spec as follows:-

OS X version 10.8.2

3.2GHz Intel Core i5

8GB 1600 MHz DDR3

NVIDIA GeForce GTX 675MX 1024 MB

When I try to edit my footage shot at 1080p 60fps the clip's will play smoothly for a few seconds and then the video starts to skip while audio continues to play smoothly. I've optimized the media and used proxy but it's made no difference. I've shut all other progamme's that were running so it's only fcp that's up and running. If i play the footage in quicktime it runs smoothly with no issues even directly from the camera, it's only in fcp that the problem arises.

Please help me out on this I'm getting really frustrated as it's stopped my editing dead and I'm pretty disapointed that I'm having this trouble on my new highly spec'd iMac!