Idle issue:
It could be many things, but possibly a clogged idle valve. The idle valve has an electronic side and a mechanical side, the mechanical side has what is known as a "wax motor" which uses engine coolant to warm a cylinder full of wax. As the wax melts it expands, closing a needle valve and lowering idle. If the small coolant lines feeding the idle valve are not moving coolant, or the passages in the idle valve are blocked, the wax won't melt until the whole of the engine is warm. You could start by checking for coolant flow through the valve.
The other thing you can check for: When the car is at fast idle, can you manually close the throttle under the hood to lower the idle? There is a thing called a dashpot on the throttle that lets the throttle close softly at the end of its travel, this can sometimes hold the throttle open slightly. If the idle lowers by closing the throttle, it is something to look at.
Temp sensor:
There are three sensors in the Capri pertaining to coolant. The sensor in the thermostat housing is the fan switch, it turns on the fan. That's its only job. There is a sensor in the head on the other side of the thermostat, pointed to the firewall, with a round pin connector - that is the gauge's sensor. Check to make sure it is connected.
The third coolant sensor is for the ECU, it is installed in the bottom of the intake manifold in the flange that bolts to the head.
Have you pulled the code to see what it is? You can do this by jumpering the test terminal under the hood, key on engine off, and counting the check engine light flashes. They are coded, there is a lookup chart on the main teamcapri.com domain along with some more information.
And welcome to TeamCapri! Hope this information is useful.
-Matt "Rocketman"