Grounding issue?

So, if it happens to the fuel gauge at the same time, it can't be ground, it must be SUPPLY side, or the (+) side of things.

And that takes me to where I was starting - the power into the cabin, or at least the fuse panel area or the ignition switch (and I have had these EXACT same symptoms in my little Eagle years ago  -exactly this - the switch supplied IGNITION but not power to the accessories - gauges, etc.). 
Depending on what keeps going - well, the ENGINE keeps running - ok, so it can't be the main power to the fuse panel as the ignition is sourced from that, too. 
Could it be at the ignition switch - where it keeps ignition feed but loses "accessory" feed (gauges, radio, blower, etc. all go through the ignition switch)

Anyway, bigbad69 - thanks for supporting what I was attempting to do - get people to figure it out logically instead of throwing things at the wall and see what sticks.
Ask "what do these things have in common".

For the alternator or charging system, nothing.

For the rest - you proved it's not a ground issue if it happens to the FUEL gauge, too.
But then we do need to consider that the cluster regulator requires a ground........ but if ungrounded the fuel gauge should peg as the regulator won't regulate (others please refer to TSM where is says regulator ground REQUiRED or you can FRY the gauges, bigbad69 already knows how that all works)

So if the cluster regulator loses ground, it stops regulating and the gauges climb, not drop. Another piece of the puzzle indicating likely feed issue.

LOGIC is your best friend in auto electric, tossing stuff at the wall to see what sticks is a way to spend many days, many dollars and still not get there.