What was found
A/P OUT OF TRIM LT CAME ON INTERMITTENTLY DURING FLT. STAYED ON FOR 30 SEC TO 3 MTS. NO PITCH TRIM CHANGE DURING ILLUMINATION. TURNED OFF AP & ACFT IN TRIM DURING DESCENT. ACFT DECENDED THROUGH SET ALT TURNED OFF AP & NOTICED ACFT WAS VERY OUT OF TRIM, INSPECTED STABILIZER SYS. INSPECTED LT & RT ACME SCREW ACTUATORS FOR DAMAGE OR WEAR, NONE NOTED. INSPECTED ACTUATORS, RODS CABLES AND CHAINS FOR DAMAGED OR WORN MATERIALS, NONE NOTED. FOUND HORZ STAB CHAIN DRIVE UNIT SHEAR PIN BROKEN. R & R CHAIN DRIVE UNIT. OPS CHECK GOOD IAW OMNI MM 27-44-05 OTHER REF'S USED 27-44-03, 27-43-01 AND 27-40-00. ACCOMPLISHED A/P OPS CHECK OF A/P NR 1 AND A/P NR 2 SYSTEMS IAW MM 22-10, NO DEFECTS NOTED.