The Bitron sensor if it's the same as the Xantia. I had the same symptoms
when the engine was hot.

Ummm, <scratches head for a while> where is it? Oh yes, around about the
thermostat housing. Undo the plug and give it a good clean, switch cleaner
from RS or Maplin is your friend here.

There was a model change in May 1999 I think. I have this later model
and had the same problem. (Which is not uncommon).
Remove the radiator grill (It is held by 2 plastic screwlike fixings at
the top. Remove these and you can pull it forward and see the plastic
clips that hold the bottom. These can be released with a long screwdriver).
If you have the same model as me you will see only 1 large cooling fan.
Start the engine and turn on the aircon and the cooling fan should run,
but I bet it doesn't.
To the right of the fan are a black and a grey connector. Unplug these
and remove the sockets from the bodywork. They are each held by 1 screw.
You will see that these parts look like a coil of wire in an open metal
casing. These are the resistors for the fan motor speed control. You
will probably find at least the black one to be open circuit.
If this is the case, it is worth replacing both anyway. Cost is about
£6.00 each. Part nos. are PT1267E3 AND PT1267A9.
Whilst you are at it, remove the cover from the relay box to the right
of these resistors, unplug the relays and check connections are clean
and give them a squirt of WD40. Ditto any other connectors you can get at.
Regards,
Ken.