Try alternating between chords (such as D to G, C to G, E to A, Dm7 to G7, Cmaj7 to C6, etc...) and playing along with a metronome at a reasonable pace. Make sure that you try to continue to keep time with the metronome even if you flub one of the chords. Try playing each chord for 4 beats then switching to the next and playing it for 4 beats and then repeating that pattern over and over again until you can play it cleanly. As you get more comfortable with the 4 beat pattern, reduce it to a 2 beat and then 1 beat pattern. This will help improve your strength and dexterity to go from chord to chord.