Ok, I know the subject of W126 fuel pumps, and fuel pump relays have been beat to death, and I have searched through all of the related threads I think. That said I'm still a bit confused about a few things.
What I'm trying to do is diagnose an intermittent "ghost" problem with the fuel and ignition system on an 88' 300SEL.
I'm starting to suspect the pump itself as the main culprit, but I want to be sure before I start throwing money at it.

Recently I've noticed that the fuel pump will get noticably louder from time to time which sometimes results in engine shutdown. The fuel pressure is good, so is the volume, and the accumulator holds. The problem is so intermittent though that I feel those tests are pretty much useless.
So I came up with the idea to connect a DVM to the X36 connector under the passenger kick panel. This way I could at least see what the voltage was going to the pump when the problem reared it's head.
Well, turns out the voltage is constant when the problem occurs (13.2-13.5V). This seems to rule out the relay. I would think that if the relay was a problem then I would see large changes in voltage.
Am I right in thinking that?
I guess a better test for the pump would be to monitor the actual fuel pressure, and flow. But I haven't come up with a practical means of doing this on the road. Any thoughts?

The wiring diagram shows two fuel pumps, but as best I can tell this car only has one fuel pump. Seems like I remember reading that certain years had two pumps?? If this car does have two pumps where is the other located?
The only one I've found is hanging directly under the fuel filter inside from the right rear wheel.

Thanks in advance for any help