The pressure sender is a common problem, and so are bad connections and breaks in the wiring.
Turn the ignition switch to the Run position. Try grounding the sender wire to the engine block; it should go full scale high. Then try wiggling the wiring while an observer watches the oil pressure gauge.
The 10 pin black and white fuel injection connectors are known to cause strange problems. Check pin #7 on the white connector and make sure that it is tight. Spread the male pin open or bend the female pin to make it close more.
See the graphic for the 10 pin connector circuit layout.