For the past month or two we've been having an ongoing issue with pressurization on our J model. The problem typically gets recognized when flying at higher altitudes, 19-20k ft, thought recently they've stated they had erratic pressurization at all altitudes. Initially the problem was erratic pressurization throughout flight, resulting in ears popping and discomfort, when at higher altitudes they received a CAB Alt high advisory because cab alt was indicating above 10k ft. We change a part, it flys 1-2 times and the problem comes back. We've changed the outflow vlv, cabin pressure controller, vacuum reg, and servo valve, some things have been changed twice to no avail. Anyone dealt with something similar/have any potential suggestions?