In addition to the sound advice above concerning the thermostat, two other possibilities come to mind.
Does the fuel guage read correctly? If not, check the voltage stabilizer on the back of the instrument cluster. Check the Bentley manual or the Brickboard for details.
Also, the temperature compensating board (plugs into the back of the instrument cluster) can be pulled out, contacts cleaned and pluged back in. Pretty easy.
Running the engine below normal operating temperature is not good for the long term health of the engine. But if you are working toward getting the problem fixed, don't worry about it.
'76 volvo 240
If you can't drive a 300E, drive a Volvo 240.
If you can, drive both.