Had a similar issue with my front doors.
First, the driver's door started locking and unlocking itself periodically as I drove along. Turned out the wiring into the door was damaged (presumably by chafing as the door was opened) and broken / exposed wires were causing the intermittent locking.
Recently my front passenger door has started to refuse to unlock. I gather failure of the lock module is not uncommon and this needs to be replaced to solve the
problem. There's a how-to
guide here on VW vortex (for Mk4 Golf but door internals look the same).
http://forums.vwvortex.com/showthread.php?1015107
Looks a bit involved but I'm going to have a go at this soon myself. I gather the lock modules are selling on eBay for a bit less than the dealer price (about £25 vs what I have been told is around £114).
HTH,
Ben