Hi James,
If the computer shuts down, it could be hardware related. It might be worth a trip to the Apple Store. Does it only happen with Premiere? Often time faulty ram can cause this, I could be worth testing by removing one stick at the time.
Also try resetting SMC:
Shut down the computer.
Unplug the computer’s power cord.
Wait fifteen seconds.
Attach the computer’s power cord.
Wait five seconds, then press the power button to turn on the computer.
Vince Becquiot
Indigo Live | Kaptis Media
San Francisco Bay Area