[ischmitz]

The speed sensor is the main timing device and the reference sensor is required to determine TDC. With an intermittent speed sensor your engine cuts out every time the signal fails. With an intermittent reference sensor you might fare better since it is not always required. It is used to correct timing every once in a while.

Both sensors can fail intermittently with age due to the heat cycling/vibration at the bell housing. Not easy to diagnose since you need to catch the failure "in the act" by having a breakout box with an O-scope. Probably a good idea to replace the sensor when in doubt before embarking on more expensive items.

Ingo