It sounds like the turntables or CDJ's are not calibrated properly.  Please make sure all CDJ effects, e.g. master tempo, etc are disabled.

If you are having issues calibrating your setup, please read the Serato DJ manual on how to calibrate.

Also check with the hardware manufacturer if there is any firmware updates or servicing needing to be done to your hardware.

TIP: If your turntables have been calibrated properly, but you are still experience slight BPM flutter you can now use Anti-Drift to remove this "natural" fluctuation. Anti-Drift is available in Serato DJ 1.9.6 - it's deactivated by default so you will need to enable Anti-Drift from the CD/Vinyl preferences in your SETUP window.