Intermittently - car cranks but but does not start. On the way to work in the morning - got off the highway & came to a stop and car died. Started right up after about 5 minutes.

So far -

New battery
Checked for codes - none
No warning on display

Ran a/c diagnostic and 24 showed only rose to 10.5 when car wouldnt start. Rose to 13.5 when running. Dropped to 13 when all accessories running.

Belt feels tight no cracks. Hear humming from rear of engine when key is in position 2.

Thinking either...

CPS
Voltage Regulator
Alternator

How do I know which one is the culprit?