79 porsche 928 4.5 how would i go about measuri g the fuel pump pressur?

On the fuel rail there is a service port that will allow you to hook a fuel pressure guage to it

You might want to have the tps throttle position sensor checked are you having these symptoms it runs fine under load and at higher rpms and when you push in the clutch or remove your foot off the accelerator it hesitates or even dies