is the electric assist fan coming on?  here in texas, on my 96, mine is
on a heck of a lot.  i would think that if it is normal temp on the
highway, the thermostat is ok.  have you checked the coolant level?
perhaps you should flush the system.

The radiator on my '98 was severly plugged - the previous owner must have
used straight tap water.  Tubes in the core were so small just a small
amount of contaminant would block them. You can let it idle enough to get
hot, then place your hand near the radiator (be careful not to touch it as
it may get very hot) and see if the temperurature is even across the entire
width of the radiator at any given distance from the top.  If it's cool in
an area it probably means the rad is plugged. You can also drain the system,
disconnect the upper and lowe hoses and see how fast water moves through it
from the cap hole.  It should keep up with a standard garden hose with out
coming out the top connector. Or, remove the radiator, place it horizontal,
fill it about 1/2 to 3/4 with water and tilt it back and foth and see how
fast the water gets from the top of rad to bottom and back.  Try it with the
rad tilted so water would only flow through one side of the rad at a time.
If it is plugged, a radioator shop may be able to rod (clean) it for about
an hours worth of time.  If it's as bad as mine was you will need a new core
or an all new radiator.

FYI, if it has plastic end caps (the top and bottom of the radiator that is
crimped to the aluminum core) and there is any discolloration, especially a
lightening of the color, around the top hose connection then the plastic has
gotten too hot and is starting to deteriorate and you should probably get a
new radiator.

Sounds like classic symptoms of a clogged radiator.  Take it to a radiator
shop to see if it can be "rodded", if not R&R radiator.