A first test would be to try to find out if it is hardware or software that is causing the stop.
When it's in it's "not working" state, what things have you tried to get it working again?
And of those things, which worked, and which didn't?
For example, can you reset the microcontroller?
Or remove, and then re-apply power? (If you have to wait a while between removal and re-application of power for this to work, please let us know how long this wait is).
Do you have a debugger attached? If so, does it lose connection when you get your "stop"?
Can you put an oscilloscope on the Vdd and Reset lines to see if either of them behaves unexpectedly to trigger the stop?
When the processor comes out-of-reset, log (e.g. to a spare serial-port) the state of RCC->CSR to see if that says why things stopped. (Once things are running properly, you need to clear the cause-of-reset flags using RMVF). What does this say?
Do you have a way to log what happens if the processor gets a HARDFAULT?
Hope this helps,
Danish
