Great.
Sounds like we have a pretty clear symptom for the diagnosis.
Based on this it is a distributor problem.
My recommendation is to position the engine at Top Dead
Center (TDC) Compression (where the rotor is pointing to
#1 cylinder). You can now remove the distributor and then
(later) re-install it in the same position.
Take the distributor to a local “auto electric” shop that has
a Sun distributor machine. The problem is the advance
mechanism is sticking and not free to move through the
advance curve. The cure is to free-up the mechanism.
After you have everything in the distributor functioning
correctly, you should document the actual advance curve
and compare it to the Porsche specification below.
"
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© Dr. Ing. h.c. F. Porsche A.G.
"
"
© Dr. Ing. h.c. F. Porsche A.G.
If out of specification, the shop should be able to help you
correct it. If not, post here and we can lead you (them)
through the process.
Best,
Grady