cooling fans

My Citreon Xantia's cooling fans are not working correctly,
When the air con is on they should work on slow speed but dont work at all, only one fan is working at full speed but it is constantly going on and off even prior to reaching normal working temperature.
Both fan are working ok as I have had both fans working on slow speed with the air con on and off.
Does anyone have any suggestions as to the cause and hopeful remedy to this problem.
thanks

Johnathon, have you sorted the problem yet?
The cause may be the control relays under the cover between the fans at the top. Another source of 'iffy' control can be the temp sensor near the fuel filter on the diesels or I think near the thermostat on petrols.
A search on the wider web for 'Bitron', the name of the control system may be advantageous.

Try this link for information on the fans system. The PSA cooling system is the same on Citroen and Peugeot
http://www.peugeotlogic.com/workshop/base/workshop.htm