DC9 MIL ACFT INSTRUCTED TO LEVEL AT FL250. PF; FO'S HSI STUCK RIGHT AFTER CTLR GAVE TURN CLRNC AND HE WAS DISTR LOOKING AT THE OTHER HSI AND OVERSHOT ALT ASSIGNMENT BY 350 FT.

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WHILE ON A SCHEDULED FLT IN A MIL C-9B WE WERE ENRTE FROM NAS LEMOORE; CA; TO NAS NORTH ISLAND SAN DIEGO. WE WERE ON OUR CLB OUT; LOCATION WAS 50 NM S OF AVENAL APCHING LOS ANGELES FROM THE N. WE WERE INSTRUCTED BY ZLA TO LEVEL AT FL250. AS THE ACFT WAS CLBING THROUGH APPROX FL230 CTR GAVE US A WESTERLY VECTOR FOR TFC. AS THE ACFT WAS BANKED R BY THE COPLT TO COMPLY WITH THE VECTOR THE ACFT HSI ON THE COPLT'S SIDE FAILED TO TRACK IN THE TURN WITH HDG FLAGS SHOWING IN THE WINDOW. THE COPLT STARTED TO CHK HIS HSI AGAINST THE RMI TO RESOLVE THE DISPARITY AND I RELAYED TO HIM OUR ACTUAL HDG OFF OF MY HSI THAT WAS RELIABLE. AS HE FOCUSED ON ROLLING OUT HIS TURN ON THE PROPER HDG; I NOTICED THE ACFT GOING PAST OUR LEVELOFF ALT OF FL250. I IMMEDIATELY VERBALIZED ALT AND TOOK THE CTL WHEEL AND NOSED THE ACFT OVER AND BACK TO FL250. I SAW A PEAK ALT OF FL253.5 AS I BROUGHT IT BACK TO LEVEL AT FL250. I RETURNED CTL OF THE ACFT TO THE COPLT AT THAT TIME. CTR CALLED US AND ASKED OUR ALT AS WE LEVELED BACK ON FL250. REST OF FLT COMPLETED WITHOUT FURTHER INCIDENT.