Indeed...Make that "sealed AND secured near the spare tire area".
Great car, I suspect an electrical issue. Could be firewall related/wiring harness. Had a similar problem with my 66 300, was the wiring harness. When it got hot enough I lost spark.Possibly the coil or elec. ign. module going bad? Pertronix perhaps?
A weak or bad coil often starts acting up when it gets hot as engine warms up.