Possibly a faulty button on the key. Easy to check using one of these (copy and paste url)
http://www.trueshopping.co.uk/product/Draper_Expert_Key_Fob_And_Remote_Control_Tester/3492/46314.html
A decent auto-electrician or locksmith should have one, but might be as cheap to buy your own.
chay, June 2010