DragonSF I thought the same, but one of my modules is a volt-meter, which is connected to the 12V rail for measurement.
If you see pin 8 going to 0V, can you trace that back and see if you also see 0V at the eurorack power connector on Pepper?
Regarding the 5V thing, a LM7805 is not recommended for this application: 300mA with a drop of 7V means 2.1W dissipated on the regulator, which is probably too much. That's why we recommend a Recom R-785.0-1.0 switching regulator instead.
DragonSF I'm using the USB 5V and the rack's 5V.
If you are powering the BBB from both USB mini and Eurorack (either directly from a 5V rail or via a regulator), the latter will prevail. If the 5V supply fails, even briefly, the BBB will probably power off (the behaviour you are observing now), unless it turns off fast enough in which case it should switch to USB power.