Why does my swimming pool pressure drop after a few minutes?

If the clarity of your water is hazy/cloudy or green, then that is the nature of a diatomaceous earth filter. They filter down so small that they build up back-pressure quicker than other filters. This should also be reflected when you backwash the filter as you will see dirty water coming out. If your pool water is not cloudy or green you may be adding too much DE when you charge the system. You may have air being introduced into the system. Lubricate the O-Ring under your pump lid with Teflon-based grease and examine the lid itself for hairline cracks. Also check the pump pot (housing) for cracks.