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Hi, my name is***** fuel pressure gauge is connected to the wrong ports. You need to connect the gauge to the line that goes from the top center port of the fuel distributor to the warm up regulator. More than likely you have a bad warm up regulator. You should measure about 3 bar on that line when the car is hot and the gauge is open.
Have you tried adjusting the mixture a little richer? Will it run well if you leave the frequency valve disconnected?
Even though you have good fuel pressure you may have poor fuel flow (volume). Have you checked the fuel tank for rust. It sounds like it may be starving for fuel after it runs awhile.
I thought it acted the same way with the valve connected or disconnected. When you connect the valve you are changing the mixture. If the fuel flow is low it could be worse with the valve connected. There is a screen that screws into the bottom of the fuel tank. If the screen is plugged with rust from inside the tank it can act this way.
You're quite welcome.