First, an air bubble; and secondly a bad pressure switch. Less likely, if you have good pressure, you have a pump issue

Do you have an old reciprocating Jabsco? http://www.defender.com/product3.jsp?path=-1%7C51%7C2234245%7C2234253&id=199750

If it is more than simple pressure switch, I would replace it with a new Surflo pump for a bunch of reasons.

Do you have an expansion tank on the system?