A CLBING DC9-30 OVERSHOOTS ITS ASSIGNED ALT BY ABOUT 250 FT.

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I WAS ON A LAYOVER AND AWOKE IN THE MORNING WITH LESS THAN A GOOD NIGHT'S SLEEP. I CALLED MY WIFE AT HOME AND PROCEEDED TO GET INTO A DISAGREEMENT WITH HER. BY THE TIME I CHKED IN FOR MY TRIP I FELT OK; BUT WAS STILL A LITTLE BOTHERED BY THE DISAGREEMENT WITH MY WIFE. THE COPLT; WHO WAS NEW ON THE EQUIP; WAS FLYING AS WE CLBED OUT OF ATLANTA; ON OUR WAY TO AKRON/CANTON; OH. WE WERE SUPPOSED TO LEVEL OFF AT FL310; AND BECAUSE THE COPLT HAD BEEN DOING A GREAT JOB FOR THE LAST FEW DAYS; I LOWERED MY GUARD AND WASN'T PAYING ATTN TO WHAT WAS GOING ON. WHEN HE TRIED TO LEVEL OFF AT FL310 THE AUTOPLT MALFUNCTIONED AND KEPT CLBING. I DIDN'T NOTICE UNTIL THE ALT WARNING WENT OFF. I IMMEDIATELY PUSHED FORWARD ON THE YOKE TO STOP THE CLB BUT BY NOW WE WERE OVER 250 FT HIGH. ATC ASKED ABOUT OUR ALT AND I TOLD HIM WE WERE HIGH BECAUSE OF AN AUTOPLT MALFUNCTION AND WERE CORRECTING. WE EVENTUALLY HAD TO DISENGAGE THE AUTOPLT BECAUSE IT WOULDN'T LEVEL OFF. CONTRIBUTING FACTORS WERE A NEW COPLT; A BAD AUTOPLT; LACK OF A GOOD NIGHT'S SLEEP; AND PREOCCUPATION WITH PROBS AT HOME. THE SOLUTION: WHEN FLYING; LEAVE YOUR PROBS AT HOME; AND REGARDLESS OF WHOSE HANDS ARE ON THE CTLS; ALWAYS PAY ATTN TO WHAT IS GOING ON. SUPPLEMENTAL INFO FROM ACN 436146: LATER IN THE FLT WE HAD TROUBLE WITH THE AUTOPLT MAINTAINING ALT. WE FLEW THE REST OF THE TRIP WITH THE AUTOPLT DISENGAGED. ALTHOUGH MAINT LATER WORKED ON THE AUTOPLT; I DON'T KNOW WHAT THEIR FINDINGS WERE. I FEEL THAT A LACK OF EXPERIENCE WITH THE AUTOPLT CONTRIBUTED TO THE OCCURRENCE BUT THAT AUTOPLT MALFUNCTION WAS THE PRIMARY REASON.