Try just strumming while you mute the strings with your fretting hand. Mess about with it. Get as fancy as you dare. I can't pick single notes as fast and clean as a lot of the players on this site, but I can hold my own with anybody I've ever jammed with when it comes to working with the bass and drums to dial-in a solid groove. When you feel confident in your ability to define a song's rhythm with your stumming, the chord changes will drop into place for you. :)