Current decrease after thawing

Dear all "patchmen"
I have a big problem with my patch clamp!!!
I used HEK cell stably transfected and 3 passages after thawing my cells, they respond bad in whole cell configuration (no response, very little current or big big current!!!).
I checked the expression and the localization of my channel... and everything seems ok. Moreover, in inside out configuration I recorded correct current with correct regulation and conductance... I checked also the mycoplasma contamination and all my cell lines are clean!!!
I'll change my drugs, my medium... and now I'm lost!!!
Did someone have had the same problem, or do you have suggestion...For information I study a chloride channel (CFTR) regulates by ATP and cAMP... and in an other lab I already performed this with the same medium composition and the cell type of cells....
Please help me!!!