WHILE DRIVING W ON M-81 HIGHWAY 5 MILES WEST OUTSIDE CASS CITY, NI. SUDDENLY LOST POWER STEERING ASSIST ENTERING CURVE.. VEHICLE DEPARTED ONTO GRAVEL WHILE TRYING TO CRANK WHEEL CCW TO GET BACK IN LANE.. THEN WHILE TRYING TO CORRECT WHEEL CW TO STAY IN WESTBOUND LANE, VEHICLE CROSSED INTO EASTBOUND LANE BEFORE REGAINING CONTROL AND REDUCING SPEED - POWER STEERING REMAINED IN MANUAL MODE UNTIL REACHING HOME - NO PRE-WARNING, POWER ASSIST DISENGAGED UPON DISPLAY WARNING FOR "STEERING FAULT", "TERRAIN MANAGEMENT", AND "ADVTRACTION". WOULD HAVE BEEN MANAGEABLE IF FAULT OCCURED ON STRAIGHT ROAD, BUT AT ENTRANCE TO CURVE AT HIGHWAY SPEED, WAS UNABLE TO HAVE ENOUGH TIME TO MUSCLE VEHICLE UNDER CONTROL. BAD SITUATION IF WIFE OR DAUGHTER HAD BEEN DRIVING. TAKING VEHICLE TO DEALERSHIP TODAY, STARTED UP WITH POWER ASSIST FUNCTIONAL ONCE AGAIN, STILL SHOWS FAULTS FOR TERRAIN AND TRACTION CONTROL. POWER STEERING SHOULD NOT DISENGAGE IF A FAULT IS DETECTED IN TERRAIN MANAGEMENT OR TRACTION CONTROL. NO INJURIES, BUT SOME WRIST STRAIN IN MANEUVERING THE STEERING WHEEL UNDER MANUAL CONTROL. NO MEDICAL TREATMENT SOUGHT.