The cameras are Sony's, they are these models
http://www.cctvproducts.com/sscdc374.html
They could very well be getting cooked inside their housings, they sit out in 95+ degree sunlight in the summer and below zero degrees in the winter. Would sunshades above the cameras help keep them from baking?
When the cameras stop working they just seem to not have any power going into them. The lenses seem to work fine, but the cameras do nothing at all.