Mine does this - when it does I just pull the big 80 amp fuse in the fuse box, and whack it back in when I need to use the car - typically the problem goes away almost immediately when I put the fuse back.

No idea why but a sticky relay makes sense - I've been meaning to get an auto-electrician to look at it but it needs to be happening at that precise time!

When this occurs, try putting the AC on for a while, then whack it into ECON mode - this often works for me with the fan turning off a few seconds later.