TEMPORARY LOSS OF ACFT CTL ASSOCIATED WITH AN AUTOPLT PROB DURING A NIGHT OP BY THE FLT CREW OF A BE100 WHEN DEP CLE.

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WHILE CLBING OUT RWY HDG AND THEN GIVEN A TURN TO 320 DEGS (R TURN). WHEN TURN WAS COMPLETED AND A CONSTANT 7 DEG PITCH UP DECK ANGLE MAINTAINED; I ENGAGED THE AUTOPLT WITH HDG AND ALT PRESELECT. THE AUTOPLT ANNUNCIATOR LIGHTS CONFIRMED ENGAGEMENT TO SELECTED OPTIONS. I DID NOT 'FEEL' ANY ABNORMAL MOVEMENT OF ACFT; BUT THE FLT INSTS INDICATED A SUDDEN UNCOMMANDED PITCH DOWN AND L TURN. I DISENGAGED THE AUTOPLT AND MANUALLY ROLLED WINGS LEVEL AND RETURNED TO A CLB ATTITUDE AND RETURNED TO ASSIGNED HDG. DURING THIS PERIOD; ATC ADVISED IMMEDIATE R TURN AND CLB TO ASSIGNED HDG AND ALT. I ACKNOWLEDGED AND RESUMED COURSE. UPON REACHING HDG AND ALT; I HAD THE COPLT RE-ENGAGED AUTOPLT AND IT APPEARED TO WORK FINE. WE WERE HANDED OFF TO ANOTHER CTLING AGENCY AND NOTHING FURTHER WAS SAID. UPON COMPLETION OF THE FLT; THE COPLT AND MYSELF DEBRIEFED THE SIT. AT FIRST; I THOUGHT I HAD MADE AN ERROR AND THAT LOOKING AWAY FROM THE INSTS IN NIGHT IMC TO ENGAGE AUTOPLT CAUSED ALT LOSS (APPROX 400 FT AND 45 DEG HDG CHANGE). HOWEVER; BOTH PLTS AGREE THAT THE AUTOPLT WAS ENGAGED FOR A FEW SECONDS BEFORE THE UNCOMMANDED PITCH AND ROLL. ON FINAL CONCLUSIONS; I DECIDED THAT IT WOULD BE BEST AND SAFEST IF I FLEW THE INSTS AND THE COPLT WOULD ENGAGE THE AUTOPLT ON MY REQUESTED SETTINGS. I COULD FIRSTLY SEE AUTOPLT ANNUNCIATORS ENGAGING TO MY REQUESTED SETTINGS AND IF THERE WAS A MALFUNCTION MY EYES WERE NEVER OFF THE INSTS AND RECOVERY COULD BE ACCOMPLISHED ALMOST INSTANTLY.