[ischmitz]

The speed sensor and the reference sensor are known to be able to fail intermittently. In other words even when they measure O.K. they can still be bad. With your tests it is entirely conceivable one or both cause what you see.

If one is completely dead (or unplugged) the car won't start since the DME has "no idea" about either engine speed or TDC timing. And the tach is driven by the DME.

In general I am not a friend of throwing parts at a probem. However, given the time you spent so far you might want to consider buying one new sensor and see if changing out the speed sensor makes a difference. If not put the old one back in and change the reference sensor. Our host sells them.

Ingo