Update:
The car still stalls occassionally (once or twice on a 10 mile run) but is running a lot better. When it stalls, it sputters and stops over a few seconds. After driving 10 miles, the plugs look like this:
Looks like they are getting too hot, which means running lean or too much advance?
I rechecked the timing: with vacuum connected to the Marelli SE100CX, I'm getting 20 degrees advance at idle, increasing to 40 degrees at 4000 rpm. I think this is quite a bit more advance that the stock Marelli S135 (which had no vac advance), but there's not much info out there on S135 (or S178) advance curves.
One thing I havent mentioned is the car is fitted with a K&N air filter (with new filter element). Could this be causing a lean condition? Do I need to go up a size on my main and secondary jet sizes?
And one final observation: I have noticed that the hot idle speed changes. I'll set it to 900 rpm one day, then the next day it will idle at 1200. It's not the linkage. I know this sounds like an air leak, but I used 1.5 bar compressed air to pressurise the engine, and apart from a small leak at the exhaust flange, it all held pressure.
Cheers
Jim