From what I have read on this forum our cars have a pressure regulator in the fuel filter. At idle the fuel rail will have about 55 psi, but as the rpm increases the pressure will drop down because of our fuel pumps output. So you may have a fuel filter problem with the regulator, but I would suspect the fuel pump you are using doesn't have the needed capacity at higher RPM. You may want to replace both with OEM parts. If that doesn't work you are back to the coils and plugs you have recently installed.
The relay is also a possible problem. I have had fuel pumps work fine so long as the engine was running cool, but as it heated up it would cause a problem with the pump and the car would die. Might be something similar with the relay is happening.