50 psi (3.5 bar) is spec fuel pressure on 325.
-the drop you are encountering is normal
If your vehicle is running rich then you need to consider the following:
-Is the car reaching operating temperature quickly? (running cold will yield rich fuel mixture)
The FP regulator is built into the fuel filter anyhow, so if you are worried about a bad fp regulator then replace the fuel filter. Make sure it's OE or Mahle.