There could be a loose washer, still, somewhere in the meter spread. It could also be debris in the PRV that needs to be cleared. Most companies will allow one free call back on the PRV to clear debris because it is known that within a week or two of installing it, there is the potential for debris to cause this problem. Not all plumbers will do this free of charge, they really aren't responsible for the stuff in your water service, but it never hurts to ask you installer.
I am concerned that you 'lowered the pressure' to the PRV to 55psi. Most domestic PRVs are preset to 50 or 55 psi and don't allow adjustment above 75 psi. Depending on the size and design of your home, you should keep it no higher than 55psi to preserve the integrity of your plumbing fixtures.
Anyways, check or have checked the valves in the meter spread, PRV cleared of debris and make sure the PRV is adjusted with a cold water valve open and then rechecked with all valves closed.