1969 dart loses spark once it warms up

check that the electronic module has heat sink compound/grease under it where it fixes to the points back plate. this helps heat transfer from the module to the base plate to stop it from overheating, which could be what you're experiencing. most 'conversions' come with a sachet of this grease and the instructions say to apply it under the module.
Heat Sink Compound, Pastes & Greases | Products for Electronics
neil.