The key for my van does not reliably work. After the van has been left standing overnight I sometimes have to press the unlock button for 5-10 minutes before it will open the van. Once I have managed to open it and start it I never have a repeat of the problem the same day but sometimes the next morning it will open and start without any problem and other times I have to go through the 5-10 minutes of trying the button again.

Does the key need reprogramming or is the problem greater than this?