FLC OF AN MLG ACR ACFT DEVIATED FROM ASSIGNED ALT WHEN THE ACFT AUTOPLT CLBED THE ACFT OFF CRUISING ALT.

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WE WERE LEVEL AT FL240 WITH A CLRNC TO FL200 AT PLT'S DISCRETION. THE CTLR TOLD US TO EXPECT A CLRNC TO CROSS 30 W OF FLAT ROCK VOR AT 9000 FT. AT THIS TIME THE FO WAS FLYING; THOUGH THE AUTOPLT WAS ON. THEREFORE; HE WAS PUTTING THE XING RESTRICTION INTO THE FMS. I WAS LOOKING OUTSIDE AT A LINE OF TSTMS AND XCHKING WITH THE RADAR ON THE NAV DISPLAY. I THEN NOTICED THE ACFT WAS NOT TRACKING TOWARDS RICHMOND VOR. I BROUGHT THIS TO THE FO'S ATTN; THEN HIT THE 'NAV' BUTTON TO GET THE ACFT BACK ON COURSE. I THEN NOTICED THE AIRSPD HAD DROPPED OFF SIGNIFICANTLY TO AROUND 215 KTS. IT WAS THEN I NOTICED THE ACFT HAD CLBED TO 24800 FT. AT THAT INSTANT THE CTLR ALSO QUESTIONED OUR ALT. WE DSNDED TO THE PROPER ALT. UPON REACHING DEST I TALKED TO THE SUPVR AT ZDC TO SEE WHAT CONFLICT IF ANY WE CAUSED. HE SAID THAT HE HAD HEARD NOTHING ABOUT IT AND STATED IT WAS 'NO PROB.' AT THIS POINT I HAVE NO CLR IDEA WHY THE AUTOPLT LEFT ITS ALT. THERE WAS NO ALT ALERT WARNING FROM THE FLT WARNING COMPUTER. I HAVE NO IDEA WHAT THE FO PUT INTO THE FMS. IT'S MY POLICY THAT SOMEONE WATCHES OUTSIDE WHILE THE OTHER HAS THE HEAD DOWN. I WAS ALSO CONCERNED WITH THE LINE OF WX AHEAD. SO MY MONITORING OF THE ACFT'S ALT WAS NOT A 'PRIORITY' FOR ME WITH AUTOPLT ON. BOTH OF US ARE NEW TO THE ACFT. I HAVE LESS THAN 75 HRS ON THE ACFT; THE FO MAYBE 100 HRS. BOTH OF US COME FROM THE LGT WHICH IS SO DIFFERENT FROM THE MLG ACFT. TO PREVENT THIS; I'LL HAVE TO NOT ONLY FOCUS ON OUTSIDE THE ACFT BUT MONITOR THE AUTOPLT AND TRY TO MONITOR WHAT THE OTHER PLT IS ENTERING INTO THE FMS. IN OTHER WORDS; DO WHAT I'VE BEEN DOING; ONLY BETTER.