Installed a SW HEI, using both pink / yellow wires to close an external relay. Good operation perfect. Problem... random restart - no spark. No correlation to fuses, switches, ... Everytime I investigate it with a meter .... all reading are good and the car starts....

Why did you use a relay? Anti-theft? To bypass the resistance wire? How did you use the yellow wire? The resistance wire is 6 volts for points. The yellow is 12 volts only in 'START'. When the key is in 'RUN' it goes back to 6 volts. HEI demands a fat 12 volts at all times. Try using just the pink wire hooked up to 12 volt ignition switched power. The resistance wire terminates at the bulkhead connector on the firewall. That is where I cut the wire and replace with a 10 gauge wire to the pink wire terminal on the HEI. If the fender is off, you can even pull the bulkhead connector pin to replace with a 10 gauge wire and re-pin it.

Describe your wiring so we know what you are dealing with.

How good are the spark plugs and spark plug wires?
You sure you running the relay from a full 12V battery source and have a good ground for the relay ?