First of all, throw away all your cells if you can do it.
Here we have had the same problem and we have solved it by several
1) Full your incubator´s water deposit with this solution:
0.2gr EDTA, 10gr. CuSO4. in 10 l of sterile H20.
Change the solution whwn blue precipitates appear.
2) In spain there is a sterilizing solution named Biosteril, wich is
used to clean the working place. It is a concentrated solution with coloidal
silver and H2O2. I am sure you can easily find such a solution.
It works GREAT. I think that this is the most important solution.
3) This is the most flexible one. Clean the working chamber once a week.
( We are not very consistent with this).
Bye and good luck.