Well, what sort of footage is this? What codec? H.264? ProRes? XDCAM? What frame size...1080p? 720p? DV? What frame rate...24, 29.97, 60fps?
Where is this footage stored? A hard drive connected how?
I will take a wild shot in the dark and guess that you are trying to edit H.264 or MP4 or AVCHD natively without converting. But that's just a wild guess...
It's DSLR footage that worked fine on my previous iMac in FCP7.
24fps. 1920 x 1080. Apple ProRes 4444 (imported directly into FCP7 using Log & Transfer).
Footage on external hard drive connected via FireWire 800 to Thunderbolt port using adapter.
I can play a clip back once and it's fine, next time it's jittery, next time it's fine -- and when I export the project out to a QuickTime movie, it plays perfectly.
ProRes 444? For DSLR footage? Wow....major overkill. The only thing you gain with ProRes 444 over say regular old ProRes 422 when it comes to DLSR footage is file size. Twice the size, twice the data rate...zero quality gain. ProRes 444 is designed for graphics with embedded alphas, and very high end camera systems like the RED or ALEXA...not a DSLR.
So that is one contributor to this. ProRes 444 on a Firewire drive...
#2...drive connected via adapter. Yeah, the new iMacs lack firewire, so you need to adapt. That MIGHT be an issue...not sure. But historically, adapters cause bottlenecks that previously didn't exist.
Thanks for the info on ProRes 444. This was the default setting for importing it -- I'm embarrassed I hadn't changed it to something smaller.
Something is still happening in FCP7 because I just imported some other footage, from tape (using Log & Capture) and it too is "wobbly" from time to time.
I'll keep working and see if I can fix it.