CE500 PILOT REPORTS AUTOPILOT HARD OVER DURING DESCENT TO 17000 FT. ACFT DESCENDS TO 16500 FEET BEFORE CONTROL CAN BE REGAINED.

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I WAS THE PF ON A NIGHT ARR TO SNA ON THE KAYOH4 ARR; HECTOR TRANSITION. WE WERE ON A PROFILE DSCNT WHEN THE CTLRS WERE STEPPING US DOWN IN INCREMENTS AS WE PASSED HECTOR VOR TOWARD DAWNA INTXN. WE WERE CLRED TO CROSS DAWNA AT 13000 FT REQUIRING A HIGH RATE OF DSCNT VIA 'STEP HOLDING.' EVERYTHING WAS ROUTINE AS WE DSNDED OUT OF 19000 FT FOR 17000 FT (ASSIGNED) DIRECT TO DAWNA; AT JUST BELOW VNE; I EXPERIENCED AN AUTOPLT 'HARD-OVER/UPSET;' ROLLING WINGS ABRUPTLY IN STEEP L TURN; (PAST 35 DEGS) IN A NOSE DOWN CONDITION. I IMMEDIATELY DISENGAGED THE AUTOPLT AS WE WERE STILL IN A STEEP DSCNT; CORRECTING TO RECOVER ALT WITHOUT OVERSTRESSING G'S ON A PULL-UP. A CTLR CAUTIONED US ASKING FOR ALT; THE SIC RESPONDED 16500 FT RETURNING TO 17000 FT ASSIGNED. (NEITHER OF US HEARD THE 1000 FT ABOVE CHIME) BECAUSE OF HI-BLOWER HEATING AND THE SUDDENNESS OF THIS VIOLENT TURN AND UPSET. AFTER THIS EVENT; THE AUTOPLT SEEMED TO WORK NORMALLY; HOWEVER; I HAND FLEW MOST OF THE ARR; JUST MOMENTARILY CHKING THE AUTOPLT. WE SQUAWK THE FMS-AUTOPLT TO ASCERTAIN THE POSSIBLE CAUSE FOR THE ABRUPT L-NOSE DOWN TURN. AS A SLOW TURBOJET IN THE HIGH ALT SYS; WE ARE OFTEN SIDE-STEPPED; OR REQUIRE HIGH RATE OF DSCNTS TO MEET FIXED-PT ALTS; TO COORDINATE WITH THE FAST TFC; FACTORS CONTRIBUTING TO THIS EVENT WERE: HIGH RATE OF DSCNT; NEAR VNE; NOISY ENVIRONMENT AND A DISORIENTING LATERAL MANEUVER VIOLENT ENOUGH TO THROW THE CREW SIDEWAYS. IT MIGHT HAVE HELPED IF I HAD REQUESTED A SLOWER SPD TO LESSEN THE UPSET SHOCK? CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE PROBLEM HAS BEEN TRACED TO A FAULTY CONNECTION BETWEEN THE ACFT'S ATTITUDE HEADING REFERENCE SYSTEM AND THE AUTOPILOT. THIS ACFT IS OVER 20 YEARS OLD AND HAS MOST OF THE ORIGINAL ELECTRONIC EQUIPMENT.