most everyone with a dsatx had this same issue. The brainstem is defective from the factory sadly.
I did my first power up today of my computer using a dsatx with brainstem. The first time I applied power, the computer went through tshree brief rounds of starting and immediately stopping. I never hot plugged the brainstem. After some troubleshooting, I found that if I removed the brainstem the computer booted. I tried switching the brainstem to a different COM header port and ran into the same problem. I then left the brainstem connected to the DSATX, but disconnected the serial connection. The computer booted fine. I then went back to attaching to a COM header, with the same result. Lastly, I tried hooking up the DSATX via a Serial to USB adapter and the computer did nothing.
I've ran into one other post where a user had similar issues, but as far as I know, Jeff never answered him.
I have this issue before, but it can easily be fixed. The braintem unstable issue (e.g. counldnt boot sometimes, no diagnostic data output) is due to the serial brainstem gets the 5v power from the dsatx J2 port.
The resolution is to connect only the TX and RX wires of the brainstem to the dsatx J2, meanwhile connects the 5v and ground wires to the harddisk power cable (I.e. the 4 pins power cable connecting the psu to the harddisk or cdrom), such that the brainstem will not get the power directly from the dsatx J2 port.
I have been using this setup for a month and it is very stable now.