MU2 PLT HAS ALT EXCURSION WHEN ENGAGING FAULTY AUTOPLT. CONFLICTS WITH MIL MU30 PLT WHO FOLLOWS TCASII RA.

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DEPARTED ROSWELL; NM; AND MODERATE TO OCCASIONALLY SEVERE TURB WAS FORECASTED BELOW 15000 FT. I WAS ON DEP OFF ROSWELL AND EXPERIENCED LIGHT TO MODERATE TURB ON CLBOUT. AT ABOUT XA15 I WAS INSTRUCTED BY ROSWELL DEP TO MAINTAIN 14000 FT AND WAS HANDED OFF ZAB APPROX 20-30 NM NW OF CHISUM VORTAC; AND RPTED CLBING TO 14000 FT. CTR INQUIRED AS TO HOW LONG IT WOULD TAKE TO REACH 16000 FT (MY FINAL ALT) AND I REPLIED 4-5 MINS. THE CTLR'S RESPONSE WAS TO MAINTAIN 14000 FT AND PROCEED DIRECT TO CORONA VORTAC; PLT NAV. AT APPROX XA20 I REACHED 14000 FT AND TRIMMED FOR ALT. I THEN WENT TO THE AUTOPLT AND SELECTED AUTOPLT ON; HDG ON; AND ALT HOLD ON. THE ACFT IMMEDIATELY WENT INTO AN UNCOMMANDED L BANK OF APPROX 20 DEGS. MY FIRST RESPONSE WAS TO DISCONNECT THE AUTOPLT BY THE CTL YOKE AUTOPLT DISCONNECT BUTTON AND LEVEL THE WINGS. MY SECOND RESPONSE TOOK MY ATTN DOWN TO THE AUTOPLT CTLR AT THE CTR CONSOLE AND I BEGAN TO CHK THE STATUS OF THE TURN KNOB AND ROLL TRIM. I THEN GLANCED UP TO THE ALT INDICATOR AND ALTIMETER AND NOTICED 14400 FT. SIMULTANEOUSLY THE ZAB CTLR CALLED NXXX TO VERIFY MY ALT WAS 14000 FT. MY RESPONSE WAS THAT I WAS IN A THERMAL AND CORRECTING TO 14000 FT. THE CTR CTLR THEN ISSUED MAINTAIN 14000 FT AND TURN R HDG 010 DEGS. I DID SO. MY UNDERSTANDING IS THAT ANOTHER ACFT (TYPE UNKNOWN TO ME) HAD A TCASII ALERT AND RESPONDED APPROPRIATELY TO ITS INSTRUCTIONS. AFTER APPROX 30 SECONDS ON HDG 010 DEGS AND 14000 FT; THE ZAB CTLR ISSUED DIRECT CORONA VORTAC CLB AND MAINTAIN 16000 FT INSTRUCTIONS AND I COMPLIED. THE AUTOPLT DID NOT OPERATE PROPERLY THE REST OF THE TRIP. THE PROB/CONFLICT OCCURRED FROM DISTRACTING MYSELF FROM MAINTAINING ACFT HDG AND ALT CTL. ADDITIONALLY; IF THE ACFT WOULD HAVE HAD AN ALT SELECT/ALERT INSTALLED THE EXCURSION WOULD HAVE BEEN ALLEVIATED.