[drolston]

Sounds like the coil breaking down as it gets hot. When cold, pull the coil wire out of the distributor and crank it to see what kind of spark jump you get. Should jump 1/2 inch to ground with blue to white spark. Then run it until it is hot and starts to stumble and try that test again. Weak yellow spark on both tests suggests condenser and points are going bad. Weak only on the test while hot says bad coil.