2007 a6 4.2 fuel problem
 

I have a 2007 a6 quattro with a 4.2 i bought the car with a rough idle and no knowledge of previous repairs. It had a rough idle but with run fine down the road with plenty of power. It had a p0304 cylinder 4 misfire. I took the intake off and tried to clean some of the valves and put it back together and the problem still is there. swapped around the coils and no change. We then replaced the injector with one from a junk yard and the miss went away but now it has a p0087 fuel rail pressure too low. It now idles fine but anything over half throttle it bogs out until you let off the throttle and gets fuel pressure again. I took it apart again and everything is hooked up. I am now thinking it could be a fuel pump but I don't know which one to try first? or is there anything else it could be?