It’s weird. Why would it work for a week and then go down the dumps? And why would it turn down the speed. Does it show the correct bpm in the player and just sound a lot slower, or is the displayed bpm on par with what you hear? And would you say that all songs sound about the same PERCENTAGE slower?
I am thinking that maybe for some reason the controller sends a (faulty) -x% pitch control midi signal?
Can you get your hands on another S2 mkII, just for a test? Play with yours til you have the problem, then unplug, plug in the other one and see if you can reproduce the problem on the other S2.
My money is still on a hardware (possbibly induced by the firmware version!) issue.
But you’ll have to take some more steps to determine this for sure.
Greetinx.