Distributor needs to be good - regardless of solution.
My '74 Targa ran good one day, bad the next. After a long, exhaustive search my mechanic checked the distributor - it had freeplay in the bearings, causing it to 'wobble' sometimes, which threw the timing out. As it only happened intermittently it took a whole for us to find it. New used dizzy fitted - car runs so much better.
I am still planning to fit a 123 unit (I can always go back to the original dizzy if I want to) and see what, if anything , improves. Also will fit a CDI+ unit to replace the original CDI.

For period correctness and a more aggressive advance curve, look for one of the following distributors. Combine with a breakerless point setup (Pertronix, Crane or equiv.) to maintain ignition timing settings.

I think this one:
0 231 159 002 - 1967 911S 2.0 L - 3BOS6C2
http://forums.pelicanparts.com/uploa...1607643440.jpg

and perhaps alternately,
0 231 159 007 - 1969-71 911S 2.0/2.2 L - 3BOS6U1
0 231 169 005 - 1972 911S 2.4L - 3BOS6U1

Confirm with charts in the respective factory repair manuals.

Difficult to replicate starting with the Marelli distributor.

Sherwood