If you've been pregnant 8 times since your tubal ligation, that sure wasn't much of a tubal ligation.
Anyway, your age will also play into the situation. Talk to a fertility specialist and see what he or she says your chances are of getting pregnant with a healthy baby at -- what age are you, in your early 40s?
I'm sorry for your loss. You should ask for doctor for some blood tests to check blood clotting, progesterone and vitamin levels.
Unfortunately there is no way to prevent a miscarriage but you can do your best to find out why it's happening and try to improve your chances of it not happening.