protein expression level of stable transfectant will change? - (Oct/03/2007 )

I have established some stable transfectants of HLA, by using different cell lines. but one problem I found is that, whatever kind of cell line it is, the expression level of HLA decreased while passaging cells, although all cells are G418 resistant.
I do not know why my stable transfectants are resistant to G418, but not express target protein? if someone knows, would you please tell me how to solve this problem?
now what I did is to get 100% G418 resistant cells, and then do single cell screening, in order to get 100% protein-expressing cells. but still, after some time, I found the expression levels dropped, why???? I am really annoyed with this problem.
could someone give some suggestion?
thanks!!

hey,
I have the same problem with my cells too, I transfected CHO cells but after a few weeks, they lose their expression while they are totally resistant,
would you let me know what do you do to solve the problem?