I have been dealing with an overheating problem on my 1.6 Liter 1996
Sentra for the past 3 weeks.  I have done a lot, but the problem
remains.  I need some ideas.  

The problem occurred on a return trip from Florida to Texas.  The temp
guage showed that it was overheating so I stopped immediately.  Water
in the overflow tank was boiling.  I put on my interior heater and
nursed the car back home.

The problem appears as boiling antifreeze in my overflow tank.  This
is the first symptom.   Later it shows as rising temperature on the
guage.  

Here is what I have tried thus far:

1. Had the radiator checked at the shop.  The tech did a flow test
with a hose.  He forced flow in the upper inlet and watched to see the
amount of flow in the lower exit.  The tech indicated that he felt the
flow was good and that my problem was probably from another source.  I
brought the radiator home without any work done on it.

2. Checked both fans.  They both turned on when the temp show in a
higher range.  They also turned on when I started up the ac.

3.  Changed out the water pump and thermostat.  I have a 120k miles on
this car.. .so didn't think it would hurt to do these procedures.
There was no change after putting in the new units.

4.  I changed the head gasket because it seemed that I was getting
bubbling in the antifreeze as I watched it with the cap off.   Also,
it seemed to make sense at this point.  I also brought the head to a
machinist.  He indicated that he didn't think that there was evidence
that the antifreeze entered the combustion chambers of the cylinders.
Since I had the cylinder head off, I changed the gasket.

I should mention that I learned a lot about timing chains, chain
tensioners, gears, and their relationships (ugh!).  It took a while
but I got it back together and working.

I'd like to know what you would do next.  I am beginning to wonder if
the radiator is still bad and may need replacement. I would appreciate
some thoughts.  Thanks.