I just experienced the loss of SHIFT + Z function

I just experienced the loss of the "SHIFT + Z" function. I did a search on the FSX forum and found others have seen this too. There was mention of it possibly relating to low memory, and I did have Active Sky running. I stopped and restarted FSX, without Active Sky running, and found that the SHIFT + Z function was once again working. Perhaps this is a memory problem (low memory). Just want to share my experience.Jim