FLYING HIS UNPRESSURIZED C310 AT BLOCK ALTS OF FL200 TO 210; SINGLE PLT DISCOVERS HIS OXYGEN REBREATHER BAG HAD FALLEN OFF ALLOWING O2 TO DETOUR PRIOR TO INHALING. NOTICES MODEST HYPOXIC SYMPTOMS BUT CONTINUES HIS MISSION PRIOR TO DESCENDING AFTER REATTACHING THE BAG. SEVERAL VERTICAL TRACK ERRORS ON DESCENT CAUSE HIM TO RETHINK THE LEVEL OF DETERIORATION OF HIS FLT SKILLS.

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WE WERE FLYING AERIAL PHOTO FLT LINES IN A C310 AT FL200. BECAUSE OF THE NATURE OF AERIAL PHOTO WORK WE WERE ON A BLOCK CLRNC FROM FL200 THROUGH FL210. AFTER ABOUT 2 HRS INFLT; I REALIZED THAT I WAS SHORT OF BREATH. AFTER CHKING FLOW RATE AND HOOK-UPS; I FOUND THAT THE RE-BREATHER BAG ON MY MASK HAD FALLEN OFF. IT HAD APPARENTLY BEEN HELD ON WITH SOME TAPE AND MOISTURE HAD SOFTENED THE ADHESIVE. I GOT THE BAG BACK ON; BUT CONTINUED TO FEEL A LITTLE SHAKY; BUT COULDN'T IMMEDIATELY IDENT ANY OTHER HYPOXIA SYMPTOMS. NOT LONG AFTER WE WERE NEAR THE END OF THE FLT; AND I ELECTED TO END THE FLT A FEW MINS EARLY. I DID NOT RPT THE MASK PROB. I ASKED ATC TO ARRANGE CLRNCS DOWN TO OUR DEST AND RELEASED THE AUTOPLT TO DSND TO THE LOWER END OF OUR ALT BLOCK. ON REACHING FL200; I PUSHED THE BUTTON TO RE-ENGAGE THE ALT HOLD; BUT APPARENTLY DOUBLE TAPPED IT BECAUSE IT DIDN'T FULLY ENGAGE. ABOUT 30 SECONDS ALTER; I LOOKED AT ALT AND WAS APPROX 400 FT LOW. I IMMEDIATELY CORRECTED; BUT NOT BEFORE SETTING OFF ALARMS AT CTR. I HAD FAILED TO VERIFY THAT THE AUTOPLT ENGAGED; AND WAS THEN SLOW TO MONITOR ALT. SUBSEQUENTLY; DURING MY DSCNT; I WAS CLRED DOWN TO 15000 FT BUT REGISTERED 14000 FT IN MY HEAD. I LATER FOUND OUT THAT TAPES SHOWED I DID READ BACK 15000 FT. ATC CORRECTED ME APPROX 700 FT LOW. EVEN AFTER LNDG; I CONTINUED TO FEEL A LITTLE SHAKY. WHILE THAT COULD HAVE BEEN EMOTION DRIVEN; I REALLY BELIEVE THAT I CONTINUED TO SUFFER THE EFFECTS OF HYPOXIA. I HAD INSPECTED THE MASK PRIOR TO FLT AND EVERYTHING SEEMED SECURE; BUT IT CLEARLY NEEDED A POSITIVE ATTACHMENT SUCH AS A WIRE TIE (THERE IS A RETENTION BEAD ON THE END). THE WIRE TIE HAS BEEN ADDED. OVERALL CONCLUSION IS THAT OXYGEN MASKS IN UNPRESSURIZED GA ACFT NEED THE SAME LEVEL OF INSPECTION AND MAINT AS ALL OTHER PORTIONS OF THE ACFT. ADDITIONALLY; ADHESIVE TAPE SHOULD NEVER BE USED AS A PRIMARY METHOD OF SECURING ANYTHING -- ESPECIALLY WHERE MOISTURE IS PRESENT.