THE HELICOPTER WAS OBSERVED CIRCLING AN ATHLETIC FIELD. DURING THE THIRD PASS OVER THE FIELD IT WAS OBSERVED TO COME TO A NEAR HOVER IMMEDIATELY FOLLOWED BY A TAILROTOR SEPARATION. THE MANUFACTURER ISSUED A SAFETY NOTICE 12/4/81 & AGAIN ON 10/4/82 STRESSING THE IMPORTANCE OF MAINTAINING ROTOR RPM & AIRSPEED. ON 10/27/82 NTSB ISSUED SAFETY RECOMMENDATIONS A-82-143 & -144 TO SUSPEND THE AIRWORTHINESS CERTIFICATE OF THE R-22 UNTIL (1) MAIN ROTOR SYST STABILITY/STALL CHARACTERISTICS & M/R RPM DECAY RATES ARE DETERMINED TO PROVIDE ADEQUATE MARGINS OF SAFETY & TO BE COMPATIBLE WITH NORMAL PLT REACTION TIMES, & UNTIL THE M/R SYSTEM IS DETERMINED TO BE IN COMPLIANCE WITH 14CFR27.661. THE BOARD RECOMMENDED A STUDY BE CONDUCTED TO VERIFY THAT ADEQUATE ENG TORQUE IS AVAILABLE TO THE M/R SYSTEM TO RECOVER RPM SHOULDA RAPID DECAY OF RPM OCCUR DURING FLT. AN AD WAS ISSUED REQUIRING RAISING THE ACTUATION RPM SETTING FOR THE LOW RPM WARNING LIGHT & REQUIRING THAT A LOWROTOR RPM WARNING LIGHT BE INSTALLED ON A R-22'S.