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This is 99% definately the wire from the sender to gauge is grounding. I've had this exact same thing happen to me on a Toyota minitruck. Since the gauge works part of the time, the wire is probably not disconnected from the sending unit, which is the most common cause. So, most likely the oil pressure sending wire has a chaff in it somewhere and is grounding out intermittently, depending on where the wire happens to rub. It's possible the wire is just barely disconnected and is bouncing between the sensor body which is fully grounded and the sensor connector, giving you the different readings.
The Toyota oil pressure gauge measures current based on how much resistence the sending unit has to ground. The gauge is never supposed to fully grounded or it will be destroyed. A fully ground gauge wire will cause the gauge needle to max out.
Once the gauge is destroyed (wire mechanism inside is bent), eventually, it will cease to read above zero or just above zero.
Before I replaced the sender, I'd carefully look at the wire. I don't think these senders go bad very often, but a wire can easily get chaffed. Plus a new sender is spendy.
The other problem is now that the gauge has been maxed out, it's likely out of calibration anyway, so it's pretty much useless as far as reliability, unless you got lucky.
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