Have you run it through an apple diagnostics disk to have it check everything? I know it sounds weird, but a looping ASD has found many a tricky thing for myself.

Also, even with using the external, PSU issues could still be lingering causing activity like this. If you have the means to try other power options, I would definitely go for it. My most recent re-hacking of an iMac didn't have these problems, but incredibly intermittent hard crashes, and it was due to a bad PSU that took forever to narrow down.

Best of luck mate,

TBird