Welcome to Stangnet. So, tell us more about the circumstances. Did you have greater than 20 psi in the past? If so, what was it at idle? Did the running problems result with the fuel pressure change? If so, it could be a few things. One, it could be a failed regulator. If it’s not adjustable, you need to verify engine vacuum, since that’s what makes it work. If all of that checks out, it could be something restricting the fuel flow; filter, something in the tank, etc.
But first, we need to know the FPR type. Remember, these engines are know vacuum leakers, since there are so many places you can have one. So, make sure it’s not a bad vacuum leak before you spend money on parts. Good luck.