what range are you using for the pitch sliders eg 6% 10% Wide etc? I find the wider you go, the less accurate the BPMs are. I set mine to as low as I need which most of the time is +/- 6%
Maybe your sliders aren't zeroing properly too? If you are having issues with two 120BPM songs staying in sync at 0 adjustment (centre indent)?
In your second paragraph you say you're using the same song on both decks but one is drifting out of sync with the other, is this with both set to the mid point (0) on the slider or just using the same BPM readout on each deck eg 126.4? This might be the issue as only having one decimal place can lead to you thinking they're both playing at the same speed but could actually be as much as one tenth of a BPM different eg 126.4 can be anywhere between 126.36 and 126.45 (or thereabouts).
I understand your frustration and there maybe something wrong with your hardware but for me beatmatching is as much about using your ears and skills as grids and tempo readouts. Good luck at the festival, play from the heart mate