B737-300 ACFT ON STAR ARR. RPTR CAPT HAD FMC SET FOR NEXT DSCNT BUT WHEN DEVIATING AROUND WX IN HDG SELECT THE FMC CHANGED MODES TO LEVEL CHANGE AND COMMENCED DSCNT. RPTR CAUGHT IT AFTER 400 FT AND RETURNED TO ALT.

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ON THE COAST 2 STAR INTO IAH; I WAS LEVEL AT 10000 FT AND 2 MI NW OF MACED INTXN. THE COAST 2 IS AN FMC ARR. I HAD THE AUTOPLT FLYING IN LNAV AND VNAV WITH 6000 FT SET IN THE MCP ALT WINDOW. THE WX WAS BROKEN CLOUD COVERAGE WITH NUMEROUS CUMULO NIMBUS ON THE ARR COURSE. I REQUESTED PERMISSION TO DEVIATE AROUND THE BUILDUPS AND PERMISSION WAS GRANTED. WHEN I SELECTED HDG SELECT TO GO L OF COURSE; THE FLT DIRECTOR COMMAND BARS WENT OUT OF VIEW AND THE MODE ANNUNCIATOR SHOWED CTL WHEEL STEERING PITCH. THIS CONFUSED ME AND I TRIED TO ENGAGE A PITCH MODE TO GET THE FLT DIRECTOR BACK AS I WENT AROUND THE BUILDUPS. AT THIS TIME; I SAW THE 6000 FT IN THE MCP WINDOW AND SELECTED LEVEL CHANGE. THE FLT DIRECTOR RETURNED AND THE ACFT STARTED TO DSND. SINCE THERE IS AN ALT RESTR AT MACED OF 10000 FT; I DISENGAGED THE AUTOPLT AND RETURNED THE ACFT TO 10000 FT. IT HAD DSNDED TO ABOUT 9600 FT. IT IS AN APPROVED PROC IN OUR OPS MANUAL TO PUT IN THE CORRECT ALT ON AN FMC ARR (IN THIS CASE 6000 FT); HOWEVER; THIS PRACTICE CAN GET PLTS IN TROUBLE IF THEY NEED TO DEVIATE FOR WX AND REFER TO MCP ALT FOR THE ALT THEY ARE CLRED TO; WHICH IS WHAT I DO ROUTINELY. IN THE FUTURE; I WILL NEVER PUT IN THE LOWER ALT UNTIL I AM AT THE DSCNT POINT FOR THE NEXT ALT.