7 miles should be enough.
Is it getting up to full opperating temp's what does the gage read halfway through your trip? (above80?)
It seems unlikley in Hawaii that running cold would be a problem... but you never know.

Also have you checked the water temp sensor (top rear of the valve cover) for propper resistance range?

__________________
-Marty

1986 300E 220,000 miles+ transmission impossible
(Now waiting under a bridge in order to become one)

Reading your M103 duty cycle:
http://www.peachparts.com/shopforum/831799-post13.html
http://www.peachparts.com/shopforum/831807-post14.html