Fuel level in tank too low allowing air to be drawn into fuel inlet. Before troubleshooting pulsing pressure be sure tank is at least half full. Filter plugged. Remember a filter can plug in an instant if there is dirt in the tank. Improper fuel line size. Something loose in tank restricting intake. Restrictive shut off valve. Bad 12V connection to pump or poor ground. Kinked or collapsed hose on suction side. Restricted return circuit. Return not plumbed into tank. Light spring in regulator, try heavy spring. Vacuum connection on regulator not open to atmosphere.