2003 Chevy Avalanche oil presser gage

The oil presser gage shows max or sometimes normal or too low. I thought problem is gage itself. But its looks okay when I turn the key to on. All gage moved same. After engine on then the gage is stay max or too low or normal. Also the moving is really quick.
anyone have any idea?
please tell me whats wrong is it?
so far I think oil presser sending unit is problem?
i havent chance to check actual il presser, yet.

thx