Strange turn signal problem

[takezo]

Hi,
My wife has a 2006 xc70. Last year the left turn signal stopped working. It would work for a few times every few weeks.A few random times it would do the fast blink. Never registered on the dash as anything wrong. Changed the front and rear bulbs. Changed the front socket. Disconnected and reconnected battery. Nothing helped.Bought a new oem stalk and it worked for 3 months....now it is not working again. Bought a second new stalk hopping that was it....no change it is still out. a few times that it worked it was raining but that may just be coincidence because it would work when it wasn't raining at times. Mechanic says he has seen it a few times before and has no idea. Dealership says they would do exactly what i did but if i wanted they would be happy to did around for a few days and see what they could find. They have no idea what it is. any help would be great as i really don't want the dealership to guess on repairs for a few days with me paying for it.