Well, sounds like its time to check the weird items.
A couple that come to mind:
The water pump, while being not old is probably a rebuilt unit.  As such,
the impeller had to come off for the rebuild and it is possible that it
isn't on as tight as you might think.  I have heard of it happening.  A way
to check is to open the tank inside the engine compartment while it is cool
and start it.  When it starts to warm up you should see considerable
turbulence in the tank. When the flow starts, rev up the motor and see if it
increases dramatically. That should eliminate the pump.

Second,  Older hoses have been known to lose the inner layer of rubber and
block the flow.  I have seen that one.  Before it is warmed up, and you have
the radiator bleeder open,  with the engine at, say 2500 RPM, do you get
pressure flowing out of the bleeder?  You have to do this before you have
pressure in the system to mask the pressure from flow.

Third:  I know this may sound insulting, but on my '84, I had a bear of a
time getting it to fill completely.  Since you have changed the radiator, I
doubt that you would have had the problem on both setups,
Again, back to the cool engine and the main tank in the engine compartment:
when it starts to warm up and flow, if you rev the motor, does the level in
the tank drop appreciably?  If so, you probably still have air in it.  I use
to fill that tank at about 3500 rpm and put the cap on while reving just to
get the coolant in.  I don't have any better suggestions for the moment, but
I do wish you luck.

Mark