BEING VECTORED FOR DELAYS WHILE ARRS WERE STOPPED FOR TSTMS OVER ARPT; ACR CREW FAILED TO LEVEL AT ASSIGNED ALT DURING DSCNT. DISCOVERED AND CORRECTED.

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FO WAS FLYING THE ACFT; DSCNT WAS MADE FROM FL330 TO FL290. IAH WAS TEMPORARILY CLOSED TO LNDG TFC DUE TO TSTMS ARRIVING FROM THE SW. FUEL WAS STARTING TO BE A CONSIDERATION DUE TO THE EXTENDED RERTE AND CLOSURE OF IAH. A CLRNC FROM FL290 TO FL280 WAS GIVEN ALONG WITH A VECTOR TO THE W AWAY FROM THE ARPT. ATC RADIO TFC WAS EXTREMELY HVY WITH ACFT DEVIATING FOR WX. THE FE WAS ON THE RADIO WITH COMPANY. I WAS ON THE RADIO COPYING HOLDING INSTRUCTIONS WHEN I LOOKED BACK AT THE ALT AND NOTICED THE ACFT AT FL274. THE ASSIGNED ALT WAS FL280. I TOLD THE FO TO GET BACK TO FL280 AND HE IMMEDIATELY DID SO. A LOT WAS GOING ON; BUT I HAVE NO EXCUSE FOR THE DEV. THE FO AND I SPOKE ABOUT PAYING ATTN UNDER ALL CIRCUMSTANCES. I DO NOT THINK THIS TYPE OF PROB WILL OCCUR AGAIN. SUPPLEMENTAL INFO FROM ACN 441151: DURING SETUP OF HOLD; WE DSNDED THROUGH FL280; OUR LEVELING REQUIRED ALT. AT FL276; I TRIED TO RECOVER WITH AUTOPLT. AUTOPLT WAS SLOW TO RECOVER AT FL274. I TURNED OFF AUTOPLT AND MANUALLY RECOVERED AND CLBED BACK TO ASSIGNED FL280. CAUSE OF DEV WAS ATTRIBUTED TO HVY WORKLOAD; TSTM AVOIDANCE; HVY RADIO COM; 360 DEG DSNDING TURN FOLLOWED BY HOLDING INSTRUCTIONS; STRONG TURB; AND VFR INTO IMC.