Is this happening when you are free motion quilting, per chance? Since it has happened on two different machines, it sounds like it might be something you are doing. FM quilting makes us pull and push on the quilt sandwich and often we push or pull to hard or fast and the needle deflects, comes down, misses the hole and "snap"! Could that be the problem? You might want to use a stronger needle, like the titanium kind also.
Michelle