Mixer tap turns on by itself

[dsaucex]

I have a combi boiler driven heating system and a mixer tap in the shower, the one with temperature control knob.
After a shower, when I turn off the tap, it doesn't go to zero position but the water stops completely at say 30/100 position (100= fully open). Then after few hours, mostly in the night when combi boiler is not being used for hot water or heating, the tap starts to run water and then i am able to close it to 0/100.
This obviously leads to some water wastage and inconvenience.
What could be the reason of this behaviour?

 

[footprints]

At a guess if it uses a conventional washer it may be swelling up in use and then shrinking when it cools. I would whip it out and see if a new washer cures it.

 

[johnny2007]

Call an exorcist...