2003 Duramax, sometimes the diesel pump does not get enoung or any pressure therefore it will not start, this might happen once a week. We have changed shaft sensors, pressure regulator, all the computer boxes etc, pump it self works just fine, and once it starts, it runs just fine. We get no error codes when it is hooked up to a machine at the shop. Any suggestions?? It has been in the shop for months now, and it is still not fixed.