Oil pressure gauge help?

[xDiesel]

My 2004 GMC Sierra is having an oil pressure gauge issue. Its the 5.3L motor and the oil pressure gauge will read 40 psi one day, then the next it will go all the way up until it can't go anymore. Then a couple days later it will be fine again. When it does go up, it will act normal, just in a different spot. For instance, it will display 80psi at idle, then it will go up with the increased rpms, and go back down and read 80 at idle. If i give it some gas and let it rev up higher, it will then drop the idle pressure slightly, bringing it closer to 40 at idle. It seems like the actual pressure is fine, and there are no check engine lights at all and no noises indicating low/no pressure. It doesn't knock or tick at all. Could it be the sending unit or the gauge itself? Its really confusing me, any help or advice would be appreciated. Thanks.