I am NOT an expert but I do suspect a genetic problem here. Since these cats are ferel they may be inbread cats several generations. It sounds like the same batch of kittens are having the same syndrome or symptoms. I would start suspecting that inbreeding may have taken it's toll.
I heard that a mother and son can have kittens toghether but then the sister from that litter can cause birth defects to appear. The cats may be susceptible to diseases other cats may be immuned to because of their inherited weakend immune systems.
A case in point: Hemophylia the disease found in humans was caused by inbreeding in family lines in ANCIENT times. It came from England from the Royals trying to carry on their bloodlines.
I would suspect your kittens maybe suffering a similar effect. There isn't anything you may be able to do. Their best chances are that they are spade and nuetered. This could prevent more kittens from suffering needless deaths.
May god bless your kitty's and take care of them.