I haven't heard of this problem before and I would suggest it's a result of age rather than anything else. Because it's intermittent it can be hard to trace, but I would be looking for a faulty connection, perhaps one that's corroded or loose. I would take it to an independent Porsche specialist, or an auto electrician.

THEY'LL happily run on unleaded without an additive. The valve seats are the same in the pre-'86 engines as those built after the introduction of unleaded fuel. But you will need to run 98-octane fuel if you want to get the best out of it.