Sorry I didn't see your post earlier and hope the problem was easy to sort, but I will post incase it may help anybody else. Get the pressure checked with a direct read oil pressure gauge to make sure the is a real problem and not a faulty gauge. You could have a bad oil pressure pump or a blocked oil way. I wouldn't run the engine until you can get it checked or you will damage the engine if the reading you are seeing is correct. Hopefully it may be just a fault in your gauge or oil pressure sender unit.
I've been dealing with this same problem for months, replaced the sending unit and still reading 0 when idle and 90-120 when accelerating. I believe your oil psi should be between 30-60 normally, my next thing i'll be doing is replacing my actual gauges. If you find the problem I would love to know what it was, till then i'll be shopping for replacement gauges.
I know with my se-r I've had the oil psi go to 120 mostly when I am accelerating, usually when I first start my car my psi can be above 90 and of course when idle goes down to 0. My oil psi before was always reading between 30-60, maybe a little higher when accelerating.
If it's dropping to 0 and going back and forth, it's the sending unit. I had the same problem in mine. I saw that you'd replaced it, but it might be work a 2nd look. If you're sure that's not the problem, I'd wager that it's the gauge itself.
The Altima SE-R's had a bad oil sending unit in them. It was supposed to be covered under the warranty, however trying to get a dealership to pay for it is like pulling teeth. Most of them will try to find some excuse as to why it is not covered.
Does your red oil light come on like if your idle dips to low roughly 500RPM the light will come on. If not and you are referring to the gauge in the dash that thing is full of crap its a wasted gauge and dont worry about it.