Sounds like the pickup coil in the distributor is opening up when getting hot. After cooling down sometimes the pickup closes and appears to indicate normal. You need to check the pickup coil in the distruibutor for continuity with an ohm meter as soon as the engine dies. Unplug the 2 pin connector at the distributor and check between the two connections on the distributor side. Should read some resistance ( continuity) I just do not remember what ohms are Normal.
Ron