Juan911,
First things first, you should check out the following website and go to page 102 of 568:
http://www.porsche.com/all/media/pdf/originalparts/en/E_964_KATALOG.pdf
This is a parts diagram and list for the 964 fuel tank, filler assembly and the vent system. Familiarizing yourself with this will help you visualize what the system is comprised of and what each piece looks like. There is no infringement issue here since it is available for download directly from Porsche.
8 miles is a short ride in my opinion for the tank to build enough vacuum to overcome the pump but maybe the shop had her running prior to your arrival to ensure the car was up and running fine. That being said, the car starting after opening the cas cap is a good sign.
From here there are two ways to go.
The first way is to pull the vent reservoir and the associated vent lines from the tank and check for blockages. Take care to look at the condition of the lines before removing them. You may discover that a vent line may be kinked or collapsed prior to removal. If this is the case then you are just that much closer to figuring out what is up.
The second way to go is to drive the car for a while, periodically stopping (keep the car running) and check under the bonnet to take a look at the tank. It is under the front carpet which should be removed for ease of inspection. Also prior to starting the trip you should open the gas cap to make sure the tank is free of vacuum. Check out how the tank looks before you go on the trip so you can see what shape it is supposed to be in. While on this trip, if you see that the tank is starting to change shape (which is fairly evident by the way the flat face above the securing strap looks) then you know the vent system is the culpret. Opening up the gas cap should relieve the vacuum and allow the tank to pop back into shape. Sometimes you can hear this.
Another sign that the tank is collapsing is that the fuel level sender sender starts bending back in towards the center of the tank. It does this a little bit from the factory but when the tank is deforming the deflection is much more pronounced.
On one car that I saw the tank would not pop back into shape. On this one I removed the fuel sender and blew hard into the tank and it popped back into shape. Then I started in on the vent system.
Keep us informed, good luck and ask questions if need be.