B732 CREW; UPON LEVELING AT STAR XING RESTR; DRIFTED OFF ASSIGNED ALT.

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AT APPROX XA30 ON JAN/XA/01; WE WERE FLYING THE CEDAR CREEK 4 ARR TO DFW. WE WERE ON THE CEDAR CREEK 309 DEG RADIAL AND HAD LEVELED OFF AT 11000 FT PRIOR TO THE TACKE INTXN AS CLRED BY ATC. SHORTLY AFTER LEVELING OFF; ATC CALLED US AND STATED THAT WE WERE ASSIGNED 11000 FT AND HE WAS SHOWING US AT 10600 FT. WE WERE IN FACT AT 10600 FT AND IMMEDIATELY CLBED BACK TO 11000 FT. I CONSIDER THE PRIMARY FACTOR THAT CAUSED THIS TO BE THAT WE WERE FLYING AN ACFT WITH A LOWER LEVEL OF AUTOMATION THAN WE ARE USED TO IN A HIGH WORKLOAD ENVIRONMENT. 90% OF THE TIME WE FLY A MODEL OF THE B737-200 WITH THE SP177 AUTOPLT (A FAIRLY MODERN AUTOPLT AND MCP; WHICH INCLUDES AUTOTHROTTLES AND AUTOMATIC ALT CAPTURE). ON THIS FLT; WE WERE FLYING 1 OF THE 2 ACFT IN OUR FLEET WITH THE SP77 AUTOPLT (AN OLDER AUTOPLT SYS; WHICH HAS NO AUTOTHROTTLES OR AUTOMATIC ALT CAPTURE FUNCTION). AT THE TIME OF THE DEV WE WERE BUSY WITH THE ARR AND DEVIATING FOR WX;. I BELIEVE WHAT HAPPENED IS THAT AS I LEVELED OFF AT 11000 FT SOMETHING DISTR ME FROM ENGAGING THE ALT HOLD SELECTOR. THE ACFT STAYED IN THE CTL WHEEL STEERING MODE IN A SLIGHT DSCNT RATE. MOST OF THE TIME; WHEN WE FLY WITH THE SP177; WE DO NOT HAVE TO PHYSICALLY ENGAGED ALT HOLD. IT DIDN'T TAKE MUCH FOR ME TO BE DISTR FROM THIS TASK SINCE WE WERE VERY BUSY AND LACKED PROFICIENCY AND FAMILIARITY WITH THE ACFT. ANOTHER FACTOR IS THE FACT THAT WE WERE ONLY AFFORDED 6 HRS OF SLEEP THE NIGHT BEFORE. THE SCHEDULED LAYOVER TIME WAS JUST OVER 9 HRS. DUE TO OUR TIGHT TURN TIMES AND HAVING TO CHANGE ACFT THE NIGHT BEFORE; WE WERE NOT ABLE TO EAT DINNER AT A NORMAL TIME DURING OUR DUTY DAY. AS A RESULT; WE HAD TO EAT DINNER AFTER ARRIVING AT OUR HOTEL. THIS; COMBINED WITH THE FACT THAT OUR RIDE FROM THE ARPT TO THE HOTEL WAS LATE PICKING US UP; ONLY LEFT TIME FOR ABOUT 6 HRS OF SLEEP. FOR ME PERSONALLY; THIS IS A BARELY ADEQUATE AMOUNT OF SLEEP; AND DEFINITELY NOT OPTIMAL.