A very low temp reading can keep a high idle....but this should pop a low temp code...
The dash temp sensor and the computer temp sensor are not the same (but you seem to be aware of that). . ....one is located near the thermostat, the other...never saw! Computer sensor has 2 wires, the other only one (according to diagram) ..though this is not clear as RockAuto tells the 2 pin can be used in place of the one pin...
www.avigex.ca/xport/enginesensors1.jpg
www.avigex.ca/xport/instrumentpanel1.jpg
The dash gauge sensor has no connection to the computer hence cannot have an effect on idle.
My temp indicator has always run low...indeed...
I'd rather suspect the idle air valve...or the throttle position sensor. A faulty TPS eventually will pop a DTC 21 or 22 code...but not always..faulty TPS (with a code) will inhibit 4th gear and TCC and cruise control.
Are you able to read the codes from the dash CheckEngineLight?
A loss of vacuum to the transmission pressure modulator (or failed modulator)
http://www.avigex.ca/xport/modulator.jpg (arrow)
will also keep a high oil pressure causing hard shifts.