Oil Gauge Problem

[Eyegore]

OK the old oil gauge was not working not even moving when power was applied. I replaced it with another gauge and it powers up fine, but when I start the motor it pegs clear over to the right side, to the highest psi then it stays there when I turn the motor off, to the right side that is. After that I click the ignition back on to power, not cranking the motor, it then goes back to the left side of the gauge to the lowest psi while power is kept on. Can anyone give me a hint of what might be the problem and a solution? Is it the sending unit that is the problem? A while back at the autoparts store they showed me a completely different sending unit that is in there now, it was a canister type.
This is for a 1978 454 3/4 ton.