POSSIBLE TFC CONFLICT WITH A TCASII TA PRODUCED BY AN ALT EXCURSION BY A MANEUVERING M82 WHEN MAKING THE TURN; REDUCING SPD AND CONFIGN AT DELMO INTXN; TX.

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AT DELMO AND 11000 FT LEVEL AND SLOWING THROUGH APPROX 230 KIAS; SLATS OUT; IDLE PWR; AND AUTOPLT ON; I MADE THE 'PREPARE FOR LNDG' PA AND LOOKED DOWN TO RESELECT THE AUDIO PANEL TO #1 XMITTER. AS I LOOKED UP; I SAW THE ALT DSNDING THROUGH 10800 FT; DID DOUBLE TAKE AND IMMEDIATELY INTERVENED MANUALLY; KICKING OFF THE AUTOPLT AND AGGRESSIVELY RETURNING TO 11000 FT (BOTTOMED OUT AT 10700 FT OR MAYBE 10650 FT). DURING THIS PULL UP; I HEARD 'TFC; TFC;' BUT DIDN'T LOOK AT THE TCASII DISPLAY (NO RA) AS I WAS TOTALLY FOCUSED ON PRECISE ACFT CTL. DON'T KNOW IF THE TA WAS THE NORMAL ONE FOR CLBING TFC OR CAUSED BY OUR DEV; OR WHERE OR HOW CLOSE THE OTHER TFC WAS. AUTOPLT DIDN'T DISCONNECT UNTIL I CLICKED IT OFF. I DON'T KNOW WHAT CAUSED IT OR HOW THE AUTOPLT STARTED TO DSND. I KNOW I DIDN'T PUSH THE YOKE. AUTOPLT WORKED FINE AFTER THE EVENT. I DIDN'T WRITE UP THE AUTOPLT TO MAINT. CTLR CAME UP ON RADIO AND HIS WORDS AND SPEECH TIMING MADE ME THINK HE WAS CALLING US ABOUT THE ALT AND THEN MID-SENTENCE SAW US CLBING AND CUT OFF HIS COMMENT. MY ONLY EXPLANATION IS THE S80 AUTOPLT HAS A LONG HISTORY OF FAILING TO CAPTURE OR MAINTAIN CAPTURE DURING RAPIDLY CHANGING CONDITIONS; ESPECIALLY LARGE RATES. THIS WAS MY FIRST TRIP (6TH LEG) IN 10 KS. SUPPLEMENTAL INFO FROM ACN 617006: WE WERE MAKING THE L TURN AT DELMO INTXN ON THE JEN 8 ARR. THE AUTOPLT WAS ON. AS THE ACFT STARTED THE TURN; IT WAS STILL DECELERATING; AND THE CAPT ASKED FOR THE SLATS. AS THE SLATS WERE EXTENDING; THE ACFT BEGAN TO DSND OUT OF 11000 FT. THE CAPT INTERVENED AT 10700 FT; AND WAS CORRECTING BACK TO 11000 FT; WHEN APCH INSTRUCTED US TO DSND TO A LOWER ALT. THE CTLR DIDN'T MENTION ANYTHING ABOUT THE UNINTENTIONAL DSCNT.