Why do I have gyro drift (with option off)?

My heading indicator keeps giving me inaccurate readings towards the end of flights. In the realism menu, I have gyro drift un-checked, and the gyro drift slider full left, but my heading indicator is still 10-30 degrees off by the end of the flight? I press the D key, and it corrects itself, but I thought that with the gyro drift option off I would not get this? I've had this option off in every version of fs, so I don't think to check it. My only indication is when the ATC starts giving me erratic vectors (worse than usual), since I am not actually flying the correct heading. Is there anything in FSUIPC, or anywhere else I may have changed? (I have the registered FSUIPC if that matters). This just started happening recently.