Why Is My Connection So Unstable When I Play Xbox LIVE?

I play Call of Duty Modern Warfare 2 on Xbox LIVE. For the connection strength, it's sometimes three or four bars. And other times it's one or two bars. I don't know why it's like this. Could someone please help me fix this so it is always three or four bars. Thanks. By the way, I have a Linksys router, I think it is a older edition since I got it like a year ago. I once got a NETGEAR wireless extender but that didn't help. My router is directly two floors above my Xbox and I do not want to move it.