Reply to BPM keeps changing!

So ive been using traktor and an s4 for a while...well a couple of weeks ago i bought 2 CMP.800's and a mixer with the idea of learning to manually beat match. So ive been playing with an USB stick and cd's only. The problem im having is that when i load a new song the BPM keeps changing for example: 128 to 125 then 130. It gets more stable towards the middle-end of the song. So i never know what the read out is. This happens about 70% of the time. Im palying with MP3'S that are 320KPS.

Anyone else having problems like this with CDJ's. Ive played outside of traktor before. Could there be anything wrong with the files im using??

I appreciate any information!