Diagnosis
The unloader valve is likely stuck or has debris in it. This valve regulates pressure by diverting water; when it sticks, pressure oscillates.
How to Fix It
Clean the unloader valve. Shut everything down and relieve pressure. Locate the unloader (where the high-pressure hose connects to the pump). Remove it, check the O-ring and spring, rinse any grit out of the seat, and reassemble.
If That Didn't Work
Your inlet water supply might be insufficient. Ensure you're using a 3/4" garden hose, the tap is fully open, and there are no kinks or a clogged hose filter.
Heads Up
Surging can also be caused by using a spray tip that’s too small for the pump’s flow rate. Try the next larger angle tip (e.g., 25° instead of 15°).