When you shut down the engine, it is normal to bleed off pressure. Any type of fluid pressure system will automatically start bleeding down when the pump is stopped, so that is not a big deal. What is interesting is when you are bleeding down from start up. I don't know what the normal PSI should be after start up, but my understanding of fluid dynamics, is that you will get a sudden burst in pressure at start up, and will slowly bleed down to a stable pressure. For a fuel injection system, I would think that anything from 30 to 60 PSI depending on the type of FI system you are using is not that far off from normal.
The interesting part is that the pressure will bleed off before you start the engine. The fuel pressure should stay high (47-48) when key is on, start up should be around the same, and then drop off. That is interesting that you are dropping at the "priming" point. I don't know if I would say that is a regulator problem rather more a pump problem. It seems that as the pressure builds it is pushing back against the pump causing the pump to slow down, almost like a backflow problem.
Just my two cents.