E39 Driver Door lock problem

Hi All,
My year 2000 528ias has developed an intermittant problem with the central locking.
When clicking the unlock button all doors but the driver's door unlock.
It will not unlock when you use the key. It will not unlock when you get in through the passanger door and press the unlock button on the centre console. When you lean over from the passangers side and pull the handle on the inside of the drivers door the "pin" on the door pops up then down again and the door still won't open.
The only way into the drivers seat when this problem occurs is to climb across the passanger seat - at 6"3 and 18 stone this is less than elegant...
After driving for a mile or two I press the button in the console again and the door unlocks.
Your help would be greatly appreciated.
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