What was found
WHILE SITTING BETWEEN RUNWAYS WAITING FOR A GATE STATIONARY ON HDG 135 DEGREE NAV DISPLAYS JUMPED TO 290 DEGREE HDGS. THEN FOR APPROX 1 MINUTE, BOTH DISPLAYS WAVERE (SLEWED ) BACK AND FOURTH FOR APPROX 5 DEGREE IN SYNCHRONIZED MOTION. AFTERWARDS JUMPED BACK TO CORRECT HEADING. NO PILOT INPUT AT ANYTIME. GROUNDED - YES. PERFORMED FMC OPS CHECK ON BOTH FMC COMPUTERS, OPS CHECKED GOOD REF B737-9 AMM 34-61-00, PERFORMED AIR DATA INERTIAL REFERNCE SYSYEM OPS CHECK, OPS CHECK GOOD REF AMM 34-21-00. THIS IS KNOWN ISSUE FROM BOEING REF 737MAX-FTD-34-17003.