There's your problem David. Porsche didn't use straight white tracer wire, they used spiral tracer, but the wire to terminal 15 is supposed to be solid red, no tracer. Somewhere along the way somebody hacked the wiring harness, and there's probably a high-resistance connection, a bad splice, a broken solder joint, or some corrosion in that path.
Now, go ahead and run a brand new jumper from the battery to terminal 15, just for temporary testing purposes. You aren't going to permanently install this, you're just going to test with it. Make sure both ends are insulated so you don't get shorts or a mini arc welder, this test wire will be unfused. Try it again and I bet you the car runs great. Then it's a question of tracing back the wire and replacing it with solid red, 1.5mm^2 cross section. That is the equivalent of 16AWG in the USA. Here is a link to the wiring diagram
http://www.pelicanparts.com/911/911_Parts/Electrical/911_electrical_82SC_Part1-2.jpg
The Germans have rules for everything and electrical wiring is no exception. In this case, the rule goes by the name of DIN 72 552, which is one of their old industrial standards which specifies what wire terminal is labeled by which number. And terminal 15 is, almost without exception in the world of German cars, the switched +12v from the battery through the ignition switch. Which makes perfect sense, inasmuch as the six pin CDI is the ignition, so you'd want to switch it on with the key.
So somewhere there is a high resistance, somewhere there is wire buggery by some prior owner or hack mechanic, who used a piece of British or USA wire to try and cover his tracks.