Make sure connections are tight and that neither the garden hose nor the high-pressure hose are kinked. Make sure that water supply to the pump is adequate with a minimum 30-psi inlet pressure. This problem can also be caused by a garden hose that is too long, or air in the garden hose line. Hold the gun trigger open for 1-2 minutes to purge air from the line. Also, be sure that inlet water temperature is not too high (maximum 100 degrees F) and that the engine oil level is not low.