ALTDEV.

Narrative

CTR HAD CLRED US FROM FL330 DOWN TO FL290. JUST BEFORE REACHING 29000 FT; ATC CLRED US DIRECT TO A DIFFERENT NAVAID. WHILE ONE WAS LOOKING UP THE FREQ AND THE OTHER PLT WAS SETTING IN THE COURSE; THE ACFT WENT 400 FT BELOW OUR ASSIGNED ALT OF FL290. THE CTLR ON FREQ 135.37 ASKED US TO CHK OUR ALT. I ANSWERED 'WE ARE CORRECTING' AND NOTHING ELSE WAS SAID. IT WOULD HAVE BEEN HELPFUL IF THE CTLR HAD GIVEN US A HDG TOWARDS THE NAVAID; THAT WAY WE WOULD HAVE KNOWN WHICH HDG TO TURN TO AND COULD HAVE TAKEN MORE TIME IN LOOKING UP THE FREQ AND SETTING IT IN. THIS WOULD HAVE LET THE PF SPLIT HIS ATTN BTWN LESS THINGS AND MAY HAVE CAUGHT THE ALT SOONER. CONTRIBUTING FACTOR WAS MALFUNCTIONING AUTOPLT ALT HOLD MODE; OF WHICH CREW WAS UNAWARE OF UNTIL THIS OCCURRENCE. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE RPTR FLIES AS FO ON THE B-727-200 FOR AN 'UPSTART' ACR. HE HAS FLOWN AS CAPT ON THE B-727 FOR OTHER CARRIERS. THE PRIMARY PROB WAS THE COMS DISTR; A ROUTING CHANGE WITH NO INITIAL VECTOR TO HELP THE FLC. IT WAS 'VERY BUSY AT THAT TIME.' THE FLC FOUND OUT ABOUT THE ALT HOLD AUTOPLT PROB IN THE MIDDLE OF THE COURSE CHANGE.