@Lewis > Could you please explain a bit better what you meant by the bpm changes on Rekordbox?
If you are using CD's in the CDJ's, or un-analyzed files on a flash drive in the CDJ's, then sometimes the tempo can vary throughout the track as it tries to calculate it within the unit itself and isn't always 100% accurate. For the best accuracy, you should only use the BPM count as a guide and use your ears for beat matching. ;)