Oil Presssure Jumping Around

I own a new to me 1997 Cobra, when I start it up the oil pressure reads just to the high side of the gauge mid-point, when I drive moderatly no change, when I start to rev up past 3500RPMs or so, the needle starts to jump around then move towards the low side of the gauge until I shift, then it goes back to normal and as I start to rev it goes through the cycle again. When I stop, the gauge sits right back at just to the high side of the mid-point...any idea what may cause this? Bad gauge? bad sending unit?

Thanks

I would assume (not to mention hope) it's the sending unit, the car seems to perform good at all RPM ranges so I'm hoping this is the all so elusive, easy and inexpensive fix!!