The crash on exporting had to do with the permissions of an Adobe folder. It was introduced by an update to Creative Cloud's application manager and was fixed within a week or 10 days. If you've updated "Creative Cloud desktop," then that issue should be resolved. The current version of that utility is 1.5.0.367. To check what version you have, open Creative Cloud, click the gear icon, select Preferences, and click the Account button.
In any case, that issue would account only for the crash on exporting. For the crashes you're hitting during normal operations within Premiere Pro, we need to know more. The system info Neil asked about can be found through the Mac menu in the upper left corner. Select "About this Mac," then "More Info," then "System Report."
To determine whether Premiere is using the GPU, go File>Project Settings>General and check what Renderer is set to.
To determine whether Ultra Key is really implicated, I'd make a copy of the whole project, load the copy, disable Ultra Key on all clips, and do the kinds of things that have been leading to a crash.
When the app crashes, what is the exact behavior? Do you get any error message? If so, please post a screenshot or transcribe the contents here? Do you get a full crash report--where you are prompted to send it to Adobe and explain what you were doing? If so, we'll definitely want to check that log file, but please don't paste the whole thing here. Adobe doesn’t let you attach a file to forum posts due to legal and security issues, but you can upload files using Acrobat.com’s Sendnow (https://sendnow.acrobat.com) or similar services (YouSendit.com, etc.)