Possible causes:

1)Loose, missing, or corroded grounds
2)Damaged tone wheel teeth
3)Loose crankshaft tone wheel
4)Static timing misset
5)Front gear train slippage
6)Misoriented engine speed or camshaft position sensor
7)Electrical noise.

Possible repairs:

1)Replace the speed or position sensor.
2)Replace the crankshaft or injector camshaft, whichever has malfunctioned.