Did you re-do the wiring when you put in the new system?

Some of the old points systems started on 12 volts but ran on 6 volts ( this can be dtected by a wire running to the start side of the starter solinoid) And Maybe the Pertronics needs a constant 12 volts....If it's like this it will do exactly what you say...sufficient spark for idle but not much more...

Simple fix is to run a new 12volt wire from the ignition circuit

GM used a ballast resistor (little white porcelian block) and they used resistor wire to step down the voltage....might want to check that out....I would be sure the new distributor requires a full 12 volts and it's not getting it....