I would suggest having your guitar looked at and make sure it is setup properly. That, 9 times out of 10 is the problem with guitars going out of tune. Also, be sure that you wrap the strings around the post at least twice on wound strings and three times on plain strings.
About the ELixer strings though, I would say dont bother. The reason they dont sound different after 6 weeks or more is that they go on your guitar dead. They sound like poop when you put them, and they sound that way unitl you take them off. If you just want consistancy, then put some on, but if youwant a good sounding string, just stick with the basic Erinie Ball, Dean Markley or whatever. If you wipe off your neck and strings with a rag when you are done playing you will extend the life of the strings and help the intonation.
MIKE