What Rick and Compart said. Plus I would add that there is a little bit of "chicken & egg... which comes first" going on. You're learning a new chord or chords, and then you attempt to play them with correct timing. However, as you make the chord changes, your fingers don't go quite where they need to, and you get lots of buzzing and horrible things happening. But if you slow down or stop to correct your finger position, you lose the timing? So what to do? Play the chords right, or get the timing down...? In my opinion, the way that you will get playing everything correctly, smoothly, fingers in the right places is to make sure you have the chord shapes down pat before trying to keep a steady rhythm. When I'm learning a new chord shape, I form the chord and then slowly strum the appropriate strings with my thumb, making sure I hear every string. If there's any buzz, I make adjustments, then strum again. If I'm playing the chord cleanly, I move to another chord, do the same thing. Then I slowly start switching back and forth, making sure every note is played cleanly. Only when I can play cleanly back and forth between chords do I then move on to adding a specific rhythm and tempo.

In the past I tried just forcing my way through the song, no matter how sloppy the chords were, but I found this just led to sloppy playing, the chords & fingering didn't magically work themselves out. I also find by mastering the chord shapes first, I progress a lot faster.

Good luck!