FYI, there is a fuel pressure check port on the fuel rail on the driver's side bank. You need a special tool to get in there and disconnect the fuel line so you can install a pressure gauge.
The ECM relay provides power to the ECM. There was a recall on the 2005-2006 Pathfinders, so you may want to check with Nissan for any outstanding recalls or campaigns. The reason you don't see the relay because it is on the back side of the IDPM. Go to NissanHelp.com's knowledge base and download technical bulletin NTB10-137B. It will give you the part number of the relay and has pictures showing where the relay is at. They show a special tool to remove the relay, but you really don't need the tool:
http://x.nissanhelp.com/forums/Knowledgebase/links/389/
I would check for stored codes. They symptoms you describe could be caused by a bad camshaft or crankshaft position sensor. Stick with Hitachi or genuine Nissan sensors if you need to replace any of them. Cam sensor failures are more frequent than crank position sensors, but if you have a stored code, it'll help you determine what one it is. Cam position sensors are on the back sides of the cylinder heads.