78 GTS US Gen light on but alternator charging OK

Hey guys
Had my 308 out a a group drive this past weekend and after one of the stops when I restarted it the Gen light stayed on.
So I figured the alternator was bad even though it had been rebuilt a couple of months ago, but I should have no trouble driving the 90 miles home on the battery which is really strong.
So here's the strange thing. The brightness of the Gen light is kinda proportional to RPM. Its dim at idle then gets brighter as the RPM's increase. Then things get weird. As the RPMs rise say to about 3K the light begins to blink. Yup blink on and off much like a turn indicator light. Now if that's not strange enough when I put a electrical load on (AC) it while its blinking, say driving at 70 mph, the gen light turns on solid. If I remove the load the light will eventually start blinking again.
After sitting a week which should eliminate a bad cell as its been my experience a dead cell will go flat after sitting and there won't be enough juice to even turn the engine over. But it spun like a top this morning.
So this morning assuming it was the alternator I started it and disconnected the battery ground cable fully expecting it to die but it didn't. It continued to Idle happily away. Just as a side note while idling without the battery connected the tach and speedo went crazy jumping all over the place. Something else in I've been having trouble with the speedo where it works for a while then stops and so on. I'm about to replace the sending unit. (don't know if that could be part of the Gen light mystery.
I tested battery voltage and got slightly over 12 with ground cable disconnected and about 13.8 with cable connected and car running at idle. Seems OK.
I searched the threads an saw mention by Paul that if a diode is bad the Gen light won't turn on with key in run but engine not started. Not sure about that but that's what I read. The Gen light does turn on solid when ign switch is turned to run but engine not started.
I checked all the fuses
I pulled the 1st two fuses on the left block as they are indicated as part of the Gen light circuit with no change to the GEN light even though the seat belt light and slow lights came on.
As I understand it the D terminal on the alternator is connected to the Gen light. (I have Paul's schematics), If the diodes
are bad the circuit will close and produce enough voltage to override the the system "virtual ground", Paul's description, and turn on the Gen light.
Other possible pertinent information.
All interior and exterior lights are LED except the 4 dash warning lights including the Gen light.
All accessories and light work.
Pertronix ignition.
My own design "blade" fuse blocks which have worked fine for months now.

Does this sound like a bad diode(s)? Any to test while on the car? I hate to pull the alternator and have it checked if its one of those "Oh yeah I know what that is".

Thanks in advance
Barry