Sorry, I should have said - it is a diesel
Sounds to me like you may have issue with air in the lines or with the SCV. This would also explain why replacing the injectors temporarily fixed the problem, because they would have bled the fuel system at that time.
The SCV is similar to a throttle for the fuel pump and controls the supply volume to the pump. For what it's worth, the SCV is usually the culprit with any fuel supply issues, the high pressure pump is rarely at fault.
Thank you Jake, you have been very helpful.
your welcome, if you run into further questions feel free to ask me.