This is one of those problems that I like "challenging".
First of all you did a really good job already trying to troubleshoot the
system.
I have a theory as to what is happening but I need to know a couple things.

First has your water pump failed before?  Did you change the pump yourself?
I am wondering if the pump failed and ground into the case (I have seen this
before) and was replaced but the case was not repaired.  This will make the
pump not operate at it's optimum.

Second, double check that the two lines attached to the radiator are hooked
up correctly.  If they are switched then this could be causing your problem.
If the flow is not in the right direction you will have problems with
overheating (learned this one with our 5-cylinder conversions).

The only other thing that I can think of is clogged radiator.  This problem
has defied logical troubleshooting before.  Basically if you have checked
everything else and you still can't solve the problem then the radiator could
be the culprit.  The PO could have added Bar's Leak which is either
restricting the flow in the rad or insulating the inside of it and not
allowing it to shed heat properly.
Hope this helps and let me know what you find.
Ken Wilford
Van-Again
John 3:16