I am also in So.Cal. and keep my cricket in a holding cage (more like a large plastic garbage can) without a dramatic loss. Some loss is to be expect with crickets over time. I don't use any heat source on them and keep them in a room that has the window slightly open for air circulation. No room heater or anything else either.
All I can think of is maybe diet (what are you feeding them), enough hiding places, moisture source, anything else being put in their container? Do you know how warm the container is getting? Maybe switch supplier unless others have success keeping crickets from them for prolonged amount of time.
Maybe others can help with ideas too as to why they are not living long.
-----
~ Juli ~
www.Polliwog-Design.com
