Another possibility to add to all the good ones already listed.
Check the roll pin that holds the drive gear on the distributor shaft. An ignition timing check can show if there's an issue, but best way to look is to pull the distributor and look. Don't forget to index the timing marks on the harmonic balancer and the location of the distributor body to the intake and the distributor shaft to the distributor body. Makes reassembly a snap.
Typical symptom of this is timing constantly moving until finally it won't run when the gear moves too far.
