[Gus Berges]

Please allow me to preface this by stating that I am stating facts solely based on my experiences and please take into consideration that where I live, the climate is constantly very hot and very humid.

I have had to replace the AC units (here they replace the evaporator and dryer bottle every time, compressor only if it is found to be leaking) on my Nissan Pathfinder, 996 C2, BMW M Z3 coupe, BMW 328is coupe, and sooner than later, my current Cayenne Turbo (2003 model). In the case of the 996 C2, the same grayish/black foam found all over the vents was the indicator that the evaporator was leaking, thus needed replacement. I tried to delay this, but it came to the point where the car would not hold a charge for more than two days. All my other cars did not have this foam falling out.
A 2005 C2S (997) that I was considering getting had this same foam all over the place. Surely enough, when I took the car in for a PPI, they right away told me that I would need to replace the evaporator. It may work fine (except for the annoying "spitting" of foam) for a month or a year, but sooner than later, the evaporator will more than likely need replacing. Perhaps it is the freon leak that deteriorates the foam and causes it to get all over the vents?
For whatever it is worth, once the repair was done, no more foam all over the place. I sold my C2 to a friend of mine three years ago and he has yet to have any AC issues. My C2 was a 99 model.