Sounds like you have a fairly adequate machine...right? Are you setting your render-queue to render odds-evens? I find on a quad-core machine it is best to just render on 3 cores...and leave the 4th for Windows OS. Some people even go so far as to open the task-manager and set affinities for each instance- I never saw any advantage doing that, so I don't anymore.
IF you use 3 cores you can setup A:M to render every 3rd frame. Start A on 1, B on 2, and C on 3.