Since the code seems to work for random periods of time, since you have already checked with different power supplies and since the code runs on an EP5 board, my guess is that there is something specific you have tweaked to run the code on this board. In the process, you may have unknowingly edited some inline assembler (most likely if you've used DTs code) and maybe missed an instruction or two somewhere. This seems like the 31 level stack is piling up but not unwinding leading to a gradual failure.

You might want to re-look at your code with this in mind.