Not Charging at all times, why not?

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So, after numerous broken/disintegrated brushes on other cars the latest car seems to be exhibiting different symptoms.  (another SS 6 model J)

I've verified a reduction in charging rate at higher rpm proving the shunt wining works. 

But today during a high spirited full throttle 50 mile road run, in 99 degrees fahrenheit temps, quite mysteriously at the 45 mile mark the amp meter went to zero at low/reasonable RPM idling to a stop and negative at a stop light on the off ramp (no drain other than ignition).  I assumed it was the brushes and raced on home only to find that the charging went back to normal when sitting in the shade idling just as it was 10 minutes earlier at the stoplight.

There was some oil and water mixed lighting "raining" on the motor generator but the wiring looked dry.  The great news is that it ran cool at full throttle for 45 miles in a strong cross wind but the charging not returning to an expected level concerns me. 

I'm going 150 miles on thursday and will be packing a crank, extra battery, small charger, jumper cables, a tow strap, and parking on a hill - unless this is an obvious sign of something you guys can think of.  All the cables are new 00 gauge and the ignition to the delco switch is 12 gauge wire from Alan at Restoration Supply.