Windows Live Mail randomly drops people out of my contact lists

I have groups set up under my contact list to whom I regularly send email.  In the past year or two, the program has been randomly dropping people out of the lists, so that sometimes when I send a group email to the 58 people on one of my lists (for example) everyone on the list receives the message and other times only certain people receive it. I never know which ones are going to receive the email. The program chooses names and sends to them only. This is driving me nuts. I've uninstalled and re-installed Windows Live Mail numerous times.  The randomness and unpredictability of this behavior is just bizarre and it turns a simple 5 minute task into an hour long one as I try to figure out who got the email and who didn't. It shouldn't be this hard. I'd like to ditch Windows Live Mail and use another program but this one does have some features I like.