MLG HAS TCASII RA DUE TO FAILURE TO RESET ALTIMETER IN CLASS A AIRSPACE.

Narrative

THE AFTERNOON FLT FROM DFW TO PSP WAS THE FIFTH LEG OF THE DAY. THE CREW WAS TIRED. THE FO COMMUTED TO WORK AND STARTED HIS DAY 13 HRS EARLIER. I STARTED MY DAY 11 HRS EARLIER. DURING OUR CLB OUT OF DFW; WE WERE DISTRACTED AND MISSED THE ALTIMETER CHANGE AT 18000 FT. THE INCORRECT SETTING CAUSED AN ALT ERROR OF APPROX 180 FT. DURING CLB AND CRUISE I USED THE PERFORMANCE MGMNT SYS (PMS) MODE OF AUTOPLT. THE PMS VARIES THE ALT SLIGHTLY TO HELP MAINTAIN AIRSPD. THE SYS NORMALLY ONLY VARIES THE ALT APPROX PLUS/MINUS 100 FT; BUT SOMETIMES GOES TO PLUS/MINUS 140 FT. IN THIS CASE; THE COMBINED DEV CAUSED A TCASII WARNING WITH ONCOMING TFC. THE TFC WAS VISUALLY IN SIGHT AND PASSED WELL CLR. ATC ASKED FOR ALT VERIFICATION AND WE RPTED LEVEL AT OUR ALT OF FL280; OUR INDICATED ALT AT THE TIME. AFTER CHKING FURTHER; I DISCOVERED THE ERROR. THE DISTR CAUSED THE ERROR AND I FEEL OUR PHYSICAL CONDITION; COMBINED WITH THE AFTERNOON FLT INTO THE SUN CAUSED THE ERROR TO GO UNDETECTED. I CONTINUE TO TRY TO BE AS THOROUGH AS POSSIBLE; AND I PLAN TO ADD A RECHK OF ALL CHKLIST DURING THE FIRST OPPORTUNITY AT CRUISE.