I have been told (by a volvo tech)that the ac evap fails due to the upper engine mount going bad and causing ecxessive movement on the engine which in turn rocks the ac lines back and forth where they enter through the firewall resulting in the evap being cracked from the movement, don't know if it is true but i have replaced my upper engine mount at 45K miles with a poly mount and there is absolutly no movement.

i have 120k miles on my car and original evap, so who knows!

Dan