I have used the WD-40 solution, and that works for a while, but is not a permanent fix by any means. My mechanic had another similar lubricant spray that also worked and lasted longer than the WD-40. He also was able to determine which door was the issue when connected to the mini diagnostic tool that probably every certified mechanic has. Than you can get the part from Fod for like $50 bucks and simply replace it/them. | 
