Sounds like you've already looked it up...
I thought there was a
sticky around here someplace that discusses the replacement process,
but darned if I can locate it. Try this:
http://forums.trailvoy.com/showthread.php?t=54225&highlight=thermostat
The gauge isn't always the most accurate thing in the world. It's just
showing what the PCM thinks you'd like to see most of the time. That
P0128 might be getting thrown due to a partially open stat that doesn't
allow the engine to heat up as quickly as the PCM would like. It may *get*
to 210, but the PCM looks for it to get to a certain temperature within
a certain time. If it doesn't, it tends to throw the P0128 code.
It's possible for the coolant temperature sensor to be flaky, but that usually
causes more erratic behavior in the gauge.
Thermostats are still a general wear item. On my older vehicles, changing
them every year was normal maintenance. These seem to last longer,
but they can still easily fail.
With a 2005, you're close to needing the 5 year coolant change anyway.
I'm overdue with mine, but decided to get therm., coolant, and hoses
and do all of them at the same time...
Good Luck!
Chris