It's baaaack! HELP

I've posted this before and really never got a definitive solution, so I'm tryin' again.
Here's the issue: Last fall my 71 Demon 340 began to mysteriously die. Driving along, tach needle drops to zero and...dead. Wouldn't re-start in a million years UNLESS I turn the key all the way off and then it will re start and MAYBE get me home.

Here's what I have done: New Mallory distrib, Accel 300+ CD type ignition(no ballast resistor needed), new Accel coil, new wires, new plugs, new alternator, regulator, starter relay, battery. There's probably more but I can't remember.

So I think it's fixed. WRONG! Put on some new rockers and rods and drove around yesterday and right before I get home, it does it AGAIN. And again. Very intermittent.

Some guys have said it's probably the ignition switch. I have one, but ot's a bear to replace.
Some guys have said it's the Mopar ammeter thing.

Somebody please at least help me diagnose this thing. It's an otherwise very nice car that I have lost all confidence in.

I'm at the point I tell a local Auto electric specialty shop today I'll pay him whatever he wants to fix it, he says "call me in early July".