Always good to measure the problem and confirm root cause before buying parts. Personally I wouldn't buy a pump until you confirmed your fuel pressure is out of spec when the problem is occuring. Pressure may drop off due to volume which points to a fuel pump issue (assuming no leaks from pump to fuel rail, injectors, etc.). If it is a failed regulator, when you restrict the return line you'll see the pressure increase to the pumps max potential which should be at least a few psi higher than spec. What is the fuel pressure specs with and w/o vacuum hose attached?

Regulators fail on the low side too when rpm's are on the high side. I forgt to factor that into the equation. Good call