There are a few things that can cause this. One is a bad pressure regulator which you already addressed, 2 is a bad injector stuck open and allowing fuel to flow into the engine. 3 a bad check valve in the fuel pump or broken fuel line. I would start by pulling the spark plugs and seeing if any of them are fuel soaked. If one or a few are and others are not then I would look at the injectors for the soaked cylinders. Just to double check the regulator crank the engine with the return line pinched and see what happens after you stop.If it holds it is the regulator. Last you can turn the key forward and then pinch the feed line off to see if that stops the pressure drop. If that works then it is the pump
Thanks Craig, I'll try those points tomorrow... Have a great night!