[Mart]

My experience with failing rotor arms and caps has been like this:

Car starts and drives great for a while.
Some time into the journey it starts to misfire at wider pedal angles. Drives ok at light pedal.
Gradually the threshold of pedal angle related misfire reduces until it just wont run at all.
Left long enough it will (sometimes) recover and I have been able to limp home in short steps after many cooling down periods.
The first time you feel a misfire at wider throttle turn around and head straight for home.

If you imagine the spark having to jump the plug gap, as long as that jump is the path of least resistance all is ok. The wider the pedal the more the compression pressure increases. As the pressure increases the energy required to jump the gap increases too. That is when the spark will try and find a weak spot in the system where the effort taken to go that way is lower that the effort taken to jump the plug gap.

Once that weaker path is found things will deteriorate quickly.

You can help matters by using low resistance plug wires (Solid core), non resistor plug caps and non resistor plugs. Plug gaps should also be conservative. (.025"). All these things help the situation by ensuring the spark has an easy time of jumping the plug gap.

If you have a really good rotor arm and cap you can use carbon core wires and maybe bump the plug gap to .027". Any steps in this direction will reveal any deficiency in the cap and rotor. I have used epoxy on caps and rotors (crab type) to help in known weak spots.

These problems are universal and independent of the distributor type. I have had problems with Helmet, Lucas and crab types.

All the above is from bitter personal experience and not just stuff I have read on the internet.

Mart