1969 dart loses spark once it warms up

WHAT DO YOU MEAN the distributor has been converted to electronic? Do you mean you have REPLACED the dist with a breakerless dist? What kind? Mopar? MSD? other?

Or has the points dist had some module like Pertronix added?

I would pull the dist connector and "work" it in/ out several times to 'scrub' the terminals and 'feel' for tightness. A magnetic breakerless dist has VERY little current (as in almost none) through that pickup circuit, so it takes LITTLE corrosion to fail

IF it is a dist that has adjustable reluctor gap (like Mopar) CHECK it with a brass feeler gauge, and check for rust/ debri/ other around the pickup/ releuctor, as well as shaft wear/ wobble/ loose/ etc

You can trigger an MSD off points (I think the white wire?) so if you still have your points dist, make sure the points are OK and set, and try it in place of