Last Post: July 8, 2010:
I hesitate to make suggestions as there could be so many possibilities.
Doubt there are shielding issues, because they tend to be stationary, but perhaps your problems could be related to actual elevator cab locations. i.e. which floor they are on or near, at the moment.
The most common problems I have heard about in hospitals were caused by 1) abused microwave ovens near nurses stations, and 2) attempting to use a preexisting DAS antenna system.
Good luck. Please share your solution when you find it. I'm sure it will be very interesting.