Diagnosis
A worn or damaged pump unloader valve. This valve regulates pressure by recirculating water. When it fails, pressure drops and surges.
How to Fix It
This requires accessing the unloader valve. Unplug the machine. Locate the unloader valve on the pump outlet side. It's a cylindrical component with an adjustment screw. Sometimes debris gets lodged inside. You can try to remove it and clean it. Often, the internal spring and seat wear out.
If That Didn't Work
The pump pistons or seals could be worn. This results in low, uneven pressure output. This means a pump rebuild or replacement.
Heads Up
Never leave the trigger locked closed for more than 2-3 minutes. This strains the unloader valve and overheats the pump. Release the trigger when not spraying.