Practically Classics Forum

Having suffered a car dying once warmed up on a regular basis, my first port of call is replacing a black rotor arm with a red one. For the sake of a fiver it's worth it, and eliminates the risk of cheap Chinese ones failing once warm.

Black ones are fine if the brass is attached to the resin without the use of that damned rivet, it's that which causes some snide "Lucas" ones to leak. Genuine Lucas are still black with a slight brownish tinge, the red ones don't leak but do shatter with alarming regularity which was one of the reason work - and several other similar facilities in other areas - gave up on that source.

The black riveted ones fall apart as well

Has your Cresta got a ballast resistor in the ignition system? Sometimes they were built into the loom, either way they can fail this way once warm/hot and "repair" themselves when they cool down. Alternatively it could be your ignition switch is failing and not passing the current the coil needs once it has been running a while and got warm.
So if the ignition key feels warm/hot to the touch when it plays up that could be your problem. As for the ballast resistor, a quick check on this is to carry a piece of wire, preferably with the right connectors to connect to battery +ve (assuming your car is -ve earth) and the other to the coil +ve and connect this when it plays up. If it then starts and runs, the fault is either in your igniton switch or the wiring to the coil including any ballast resistor fitted.

Hope you get to the bottom of it!