Several different or combined problems may be causing your cultures to crash. A common cause of cells crashing is the destruction of the medium you are using by light. Both incadescent and fluorescent light can destroy the medium. The time it takes is dependent on the medium. A medium that is phenol-red-free and contains HEPES is destroyed by the light in the hood or laboratory in as little as 2 hours. The light breaks down riboflavin and to a lesser degree folic acid, B12 and folinic acid, trytophan and tyrosine form dimers and the result of these changes is that hydrogen peroxide is generated, which initiates the production of free radicals. Phenol red helps protect the medium by absorbing the same wave lengths that destroy the medium and serum helps protect by breaking down the hydrogen peroxide. HEPES increases sensitivity 3 fold. Taking out the bulb in the refrigerator or protecting the media with black plastic helps. What you describe can also be due to mycoplasma contamination of your cells. Mycoplasma rapidly utilize arginine, induce acid formation and alters metabolic processes. You also need to be sure your water is of high quality. Bacteria in the water system can produce toxins and the water may be contaminated with heavy metals, some of which are very toxic. If your cells are free of mycoplasma and you are sure your water is of good quality, the problems most likely are due to light degradation of your media.