The regulator is a possibility but most problems with these ox66 motors have been related to the high pressure electric fuel pump get weak or quitting all together. Only other thing I can see related to a pressure drop when running is voltage at idle. I would be very careful running this wide open until I found the problem.
“The world will not be destroyed by those who do evil, but by those who watch them without doing anything” Remember what He has done and remember that He is not done yet! GOD IS GOOD