im experiencing freezes in fcpx too. only when exporting (very rarely when working with) project which includes h.264
when exporting pure xdcam/prores there is no problem, but when i export a project with various type of h.264 material i always get freeze. its never at the same spot either.
interesting find is, when i go into menu and hover over the export, computer stalls for a couple seconds on this big project before offering export options. when i open this same project on regular imac, there is no slowdown.
also, when creating proxies, there is this h264 material where i always get glitches, but not on regular imac
my take: there are still some glitches with h.264, which demostrate when you export (not in regular playback in safari etc). i also think this is due to no quicksync on my hack, or not proper working quicksync on ppl with igpus (read many threads where igpu fixed freezes). sadly exporting from compressor fails, although it doesnt freeze the system. would be great if someone would replicate the same way its done on Imac Pro. Than all the ppl would really need is dedicated GPU encoding/decoding/processing as in real imac
this is all very sad when 580 should be kinda native and with so many ppl reporting success (although most of them dont use fcpx...)
note: im now trying 980ti for couple days, yesterday export of this same project (once) was without problems, so far i blame gpu