Fluctuation in pressure can be caused by debris in the filter. Start by cleaning and back-flushing your system. Pressure problems can also be caused by seals that are leaking. Check the area around your pump for dripping water, which could indicate a problem with a seal.
Make sure your pool is full enough that the skimmers don't suck air. Make sure your basket lid is tight. Open your valve on the filter until water comes out and then watch the pressure. Make sure to bleed all air before turning back on. You probably have air getting into it.