Hello, Had my p/s pump rebuilt a year ago, lately I've noticed that at idle and low rpm that I have no p/s, checked belt, fluid etc, A friend of my told me that they might have not calibrated it right ? Any thoughts. Thanks in advance

sounds like a belt issue.
unless you want to just start throwing money at it, i would take it to someone who can power flush it and do a pressure test.

Pump is probably the issue.