PLT stated that the ACFT pitched 20 deg nose down without warning while in cruise FLT at 1,200' and 120 kts. Following recovery, the ACFT required additional RT pedal to stay in trim and it flew about 10 deg nosed down in lvl FLT. FLT diverted to the OPR's base and landed successfully. Examination revealed that the RT horz stab had separated just outboard of the tailboom. Metallurgical examination of the separation revealed multiple fatigue origins in the areas of both the forward and aft top spar cap rivet holes and that the crack in the aft spar had been present for some time. Previous failures in this area had been attributed to working rivets causing stress concentrations in the rivet holes. Prevention of this type failure is effected by replacing the affected rivets in a timely manner. Maintenance records revealed that the left horizontal stabilizer had been repaired on 3 prior occasions, but there were no records of repairs on the RT stabilizer.