I'd bet that the issue is where the wire connects to the sending unit. It's a relatively easy fix, disconnect the wire, pry the contact up with a small screwdriver, clean, and re-connect. I get nervous when my oil pressure gauge reads zero. (After installing my motor, I was very concerned, since it would do as you describe. It was well worth the peace of mind for a quick fix.)
A better gauge is of course better, but it's a good thing to at least have the stock unit working.
A better gauge is of course better, but it's a good thing to at least have the stock unit working.