I have the same problem with my Renault 5. The car doctor said it was a 'faulty switch on the stem' and will cost £100 for a new part, grrr! I'm off to the scrap yard to see if i can get a cheaper one. Dunno if it's the same problem on your car but it sounds like the same symptoms. You might also want to check the electrics to see if it's just a loose wire.
Good Luck!
Beth T (funki8itch@hotmail.com), February 2005