On September 18, 1995, at 0940 central daylight time, a MBB BO-105C, N91070, registered to, and operated by Petroleum Helicopters, Inc., as a 14 CFR Part 91 flight, experienced an in- flight uncommanded pitch down near Lafayette, Louisiana. Visual meteorological conditions prevailed and a company flight plan was filed. The helicopter was not damaged and the commercial pilots were not injured. The flight originated from Lafayette, Louisiana, about 2 hours 10 minutes before the incident. The pilots reported that, while over "Fast Lane," a visual check point on the highway approach to the Lafayette Regional Airport, Lafayette, Louisiana, the helicopter "abruptly and violently pitched nose down." It took both pilots pulling aft on the cyclic to level the helicopter. The pilot-in-command estimates the helicopter pitched down 90 degrees; however, less than 100 feet of altitude was lost. Once control was re-established, the pilot-in-command saw that the "hyd block" light was illuminated and the Mast Moment Indicator (MMI) limit light was on steady. The helicopter was landed at the Petroleum Helicopters, Inc., heliport on the Lafayette Regional Airport, without further incident. An examination of the Tandem Hydraulic Unit (THU) by Petroleum Helicopter maintenance personnel revealed that, the hydraulic system had switched to the number 2 system. After rotating the main rotor blades, the hydraulic system switched back to the number 1 system. The THU was removed and sent to the manufacturer for examination and testing. An examination and testing of the THU was accomplished on November 2, 1995. The number 1 hydraulic system did not function properly. The hydraulic pump had a loss of pressure and the hydraulic pressure switch did not indicate a loss of pressure. The hydraulic pump was found to be leaking internally. A nick and an abrasion were found in the edge of the longitudinal control slider. This control slider controls the oil flow from the actuator to the return line. Metallic chips were found in the hydraulic high and low pressure filters. A metallic chip was also found in one of the grooves of the longitudinal control slider. The electrical portion of the "jam-proof system," which should activate the back-up number 2 hydraulic system in case of a control slider jamming, only worked randomly. The connector was heavily corroded and the magnetic core of the solenoid valve had an internal short circuit. See enclosed manufacturer's report.