Are you right handed? I am and my right hand is larger/more muscular and fatigue for the weaker hand might be occurring. Or, with a normal key layout you might have more keystrokes on the left side - have you tried a Dvorak or split keyboard. Finally, is your posture and wrist angle/support the same as for your right hand? I had wrist soreness due to the extreme thinness of a previous apple/Mac keyboard (wrists were bent upwards).