Back in 2001-2012 MANY of us VX owners were having the same overheating problem...went thru the steps you did, including the RD racing radiator, but many (myself included), found out it was the fan cluth and the fan blade. Isuzu redesigned the fan blade too.
This thread is in second link eternal21 listed in post above:
http://www.vehicross.info/forums/sho...avis+fan+blade