@Brendan iDig 2D has been absolutely spot on for me for the last 6 years, there must be something out in your settings? When using the slope feature, I have made the mistake of having the slope going the wrong direction before, I've also put the slope angle in as a percentage not a degree! All of that will throw the machine wildly off course even if you are only using it by benching off an existing part of the dig rather than setting it to the laser.
Going back to basics, if you have it on some relatively level ground and hold the bucket on a fixed point. Zero it and see if it still reads zero in any other position you put the boom. If its out, then something is wrong with the setup.
I honestly wouldn't be without my system now - I'd have it over a tiltrotator any day.