The fact that the fuel pressure drops to zero instantly indicates probable bad check valves in both the regulator/filter and fuel pump. The attachment, copied from the 04 FSM, justifies that conclusion.
Unless the pressure measurements are really inaccurate, I have to think that the regulator and pump are probably bad, or they were not the correct parts to begin with, or they were not installed correctly. There are ways to check them out individually, but IMO a visit to a professional who has the correct equipment may be a necessary expense for the needed accurate diagnostic help. Good luck!