1 - When you notice it running slow, check Task Manager to see which apps are taking up all your memory
2 - It may not be actual programs taking up your memory that causes the slowness, it could be a program that automatically updates in the background(i.e. - Dashwire) that could cause it to lag a bit because it sucks processor speed up a bit.
3 - Skyfire is a memory whore...but it shouldn't be an issue unless you have multiple instances of the same app running
4 - Any apps that you've installed since you got the phone, check to see if any of them try to run at start-up. On occasion, Photo Contacts will start to bug out on me and never COMPLETELY load until I physically open the app. After that, it's fine.
Those are the only things I can think of off top. Hope one of those helps.