When you have a slow turn failure, does the engine actually turn? This can be seen by trending engine speed during the failed start. If it is slow-turning, but never getting to the next sequence…starting, it is likely a failed speed sensor (ST173). We have had this happen several times. The two speed sensors on the cam gear (ST173 & ST174) are treated as primary & secondary, respectively. If the primary fails, you will not get an alarm…it just will not get past slow turn. The easiest way to troubleshoot this is to have eyes on the ESM while a start is initiated, and watch the lights to see if there is a pulse from the sensor. These sensors can work intermittently, which may explain why it starts some of the time and not others. Even better is to put an oscilloscope on both sensors and compare the signals. This will sometimes show a weak signal (smaller square wave), even when the sensor is working.
If you find a bad primary sensor and want to start quickly, just swap the wires with the secondary and use it for starting…then change the sensor when it is convenient.