It might also be your external harddrive. If you can try switching to Firewire (to me it seems more stable than usb 3.0). Also, make sure all your footage stays on one HDD (+ render files)
Premiere supports ProRes for Macs, right? I think the best codec for you would be ProRes 422 HQ or LT if your machine cant handle it.
Another thing you could check is software incompatibility. Turn off all other programs in the background including browsers etc.
