I realise this is pretty old now, however in case anyone else is looking for the answer, here's what i did.
My fob started being intermittent and pressing the button down harder, and moving it about worked for a while.
After getting the courage to open the key, I found that the little IR LED had one of it's legs snapped off.
Simple fix, go to your local electronics store and buy an IR LED (usually about 50p). Take a soldering iron and CAREFULLY remove the current LED. Put the new one in it's place and solder it up. Simple! :) Takes about 10 minutes.
Here's a tip too, if you dont want to solder it, run out and test it, then have to come back again, get your webcam or digital camera. Point the key at it and press the button, if you can see it flashing, you've solved the problem!
Best of luck to anyone who has this issue! :)
Davey Carter, July 2009