Keep the temperature constant and in the 70 degree range ....
I am not sure the reason for the above advice. Crickets typically thrive at temps in the 80's F, and peak out their growth rates at temps in the upper 80's. Temps in the 80's do not contribute to premature death in any way, except to grow the animal to adulthood quicker than temps in the 70's. They also can handle temps that fluctuate as one would expect in a normal summer day-night without any adverse problems.
Could be several things. If there is excessive moisture, or you are not replacing the moisture source every 2-3 days, mold and bacteria can become an enemy. Smaller crickets also will succumb quickly if fed too much fruit, melons, etc, as in high moisture content, than if fed greens. For lack of a better description, they get too much diarhea. In example, watermelon or cantelope will kill off small crikets in just a day or two, as they spend too much time eating it, and not other available dry food, while larger crickets seem to manage it better.
In addition to a vegetable-moisture source, you can raise crickets on inexpensive cricket or worm food available through many feed stores. Only need scatter a layer of it in the bottom of their cage. It needs to stay dry is all. You would then want to gut-load them better in the hours before feeding your chams, but there is no need to raise them (the crickets) on the expensive stuff all the time. Good luck.
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