[Mike Taylor 3601]

Are the lifters you all are having clips coming out full travel lifters,being they have the wire clips I am going to guess they are. Over the years I have worked on cars that lost the clips out the lifters, I believe a lot of the time valve float is the cause of clips coming out,when engine goes into valve float it starts compressing lifters and or gets slack between pushrods, lifters and causes inside of lifter to bang against clip and pop them out.
Like several have said put snap rings in them will stop it and even better switch to short travel lifters,I have shimmed regular lifters to reduce travel, you have to have .015'' travel to be legal.
May need to up spring pressure also.
Mike Taylor 3601