MLG OVERSHOOTS ASSIGNED ALT CAUSING LESS THAN STANDARD SEPARATION.

Narrative

DURING THE CLB ENRTE FROM TUS TO LAS OUT OF AROUND FL230 WE WERE ASSIGNED FL290 FOR XING TFC AT FL310. WE WERE FILED AT FL350. AT AROUND FL260 WE WERE GIVEN DIRECT TO BOULDER CITY VOR. THE F/O WHO WAS FLYING WAS WORKING THE FMC TO SET UP DIRECT (RNAV) AND I STARTED LOOKING AT THE CHART TO CHK THE APPROX HDG AND MILEAGE/TIME ADVANTAGE. THE ALT WARNING HORN SOUNDED. I LOOKED UP TO CONFIRM AND CALL OUT 1000' TO LEVEL AND SAW THAT WE WERE CLBING THROUGH FL290. ALSO FL300 WAS NOW SET IN THE MODE CTL PANEL. WHILE LOOKING FOR OUR TFC (WHICH I SAW RIGHT AWAY AT OUR 2 O'CLOCK); I CONFIRMED OUR ALT WITH CENTER. BY THE TIME I MANUALLY REVERSED OUR VERT DIRECTION WE HAD REACHED ABOUT FL297. CENTER CALLED OUT TFC AT 2 O'CLOCK AND 6 MI WHEN WE WERE ABOUT FL295 AND DSNDING. BOTH THE F/O AND I BELIEVE WE CHKED THAT THE CORRECT ALT HAD BEEN SET IN THE WINDOW (MCP) AND FEEL THAT A ELECTRICAL OR MECHANICAL MALFUNCTION CAUSED THE MCP PANEL TO CHANGE FROM FL290 TO FL300. THERE IS A MANUFACTURER'S RED BULLETIN ON NONSELECTED MCP DISPLAY CHANGES; SUCH AS ALT SETTINGS. BOTH OF US ARE NEW TO THE MLG AND ITS FMC SO IT IS A BIT OF A DISTR AT THIS POINT. ALSO THE EXCELLENT CLB 2000+ FPM CAUSED US TO NOT BE AS AWARE OF OUR ALT AS WE SHOULD HAVE BEEN TO PREVENT THIS UNINTENTIONAL ALT OVERSHOOT.