Thanks for everyone's help.
The video clip plays in camera, so it's not corrupted at source. It's just in the process of transfer to my mac and always hits a 4.29gb wall. I'm doing a direct transfer from camera, and I think this is the problem.
I don't know how to check the file size of the video clip in camera, but I'm postive it's over 4.29 gb.
It's about 10 mintes long, and only plays for 5 mins before freezing - after transferring. So my guess is its about 8gb in size, and for whatever reason the Xt3 transfer process limits it to 4.29gb, even though the firmware was updated to record clips longer than this.
Based on the other thread, listed above with the same problem, I've ordered a card reader so hopefully that will solve the problem!
I'll report back later with an update.