Just to reiterate - while elecrtonic ignition modules like the Pertronix are a great improvement to many, they cannot compensate for wear and tear in the basic distributor unit. If your distributor has issues, it may need to be replaced or rebuilt; for rebuilds I would personally recommend Dr Dan. But another option that has come on the market relatively recently is the 123 combination distributor + electronic ignition all in one unit. It is more expensive than a Pertronix unit, but (much) cheaper than a new distributor + Pertronix (or other electronic ignition module).
On my car, I first rebuilt the distributor, then added electronic ignition and then, based on the positive reviews, sprung for the 123 unit anyway, and it made a marked improvement in running characteristics. Sorry to hijack this thread, but since lots of people are following it, I thought it would be good to call out this alternative. Of course, no relationship with 123 manufacturer (even though they, like me, are Dutch).