Don't peat and driftwood soften the water? Or was that only lower pH? I'm not sure, but the idea of softening and peat seem to click in my mind. I wouldn't go for chemical softeners either, though.
Your problems are unknown to me though, my water is *really* soft. The kind of soft that's prone to pH crashes, supposedly. Hasn't happened yet, thankfully.
Also, check if the hardness is due to your tap, or if its something in the tank making it that way. Certain rocks or shells can raise the hardness (you can check if they're prone to it by the vinegar test).
But as long as its stable and your fish don't seem to have a problem with it, I wouldn't worry about it too much. Certain fish do better in hard water, like said above, swordtails for example I think.