ALTDEV ALT OVERSHOT IN CLB.

Narrative

THIS AIRPLANE WAS HAVING NAV AND AUTOPLT PROBS AND WAS ON AN IMPORTANT MISSION. I DELIVERED ANOTHER AIRPLANE TO LAS VEGAS SO THE MISSION COULD BE COMPETED. MAINT WAS PERFORMED; AND WE FILED A FLT PLAN HOME. DURING CLBOUT; #1 VOR FAILED (DEV BAR LOCKED UP 'CTRED'). #2 VOR WAS SELECTED; IT WORKED. MY COPLT WAS PROGRAMMING THE OMEGA NAV. TFC WAS RPTED TO US AND WE WERE LOOKING INTO CLOUDS SEARCHING. THE ABOVE NAV PROBS OCCURRED APPROX THE SAME TIME AS WE APCHED THE VOR AND OUR ALT. I MISSED (FAILED TO NOTE) ALT FOR 45 SECONDS AND RECOGNIZED MY ERROR AT THE SAME TIME ATC QUERIED ME. I REDUCED PWR AND RETURNED TO FL200 (ASSIGNED) RAPIDLY 15 SECONDS. THE CREW WAS TIRED; DID NOT TRUST THE EQUIP (NAV; AUTOPLT). INCIDENTALLY; THE AUTOPLT HAD BEEN SELECTED ON ABOUT 2 MINS BEFORE THE INCIDENT AND HAD FAILED (TRIM FAILURE) ABOUT 1 MIN BEFORE THE INCIDENT. WHILE ATTEMPTING TO CORRECT SEVERAL PROBS AND SEARCHING FOR TFC I FLEW THROUGH MY ASSIGNED ALT. HOW TO FIX IT: ALT ALERTER AND BETTER EQUIP. DON'T FLY TIRED.