Why does my pressure surge - Dewalt DPD3100 Gas Pressure Washer

Common problem across brands. When you spray water the drop in pressure triggers the motor to take the hose pressure ~ 30 psi and boost it to ~ 3000 psi. Water coats these parts with calcium (hard water) or other junk and water contains bacteria which causes slime and scale. Once that pressure switch / valve is sufficiently clogged it doesn't move smoothly so it can fluctuate between sending a yes / no signal about whether you are squeezing the trigger or not. This causes the pulsing.

EZ test: take the the tip off the wand, start the unit, squeeze trigger - should run continously with no problem. If it does for sure you need to watch a video on dissembling and cleaning the valve body. It is not hard but you must work carefully to avoid losing parts especially springs and take digital pictures to record the order for reassembly.