POTENTIAL CONFLICT ENSUES AS A CPR FLC LEAVES THEIR ASSIGNED ALT DURING A CLRNC INTERP PROB WITH CTLR AT ZOA; CA.

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DSNDING ENRTE/TRANSITIONING TO APCH PHASE INTO MRY. ZOA WORKING US ON FLT PLAN; THEN VECTORS; THEN DIRECT TO ANOTHER INTXN. CONTRIBUTING FACTORS: TURNS; GETTING ATIS; MULTIPLE ENRTE (CLRNCS) WITH MULTIPLE CTLRS. CTLRS BUSY WITH MULTIPLE ACFT. A NORMAL SCENARIO BUT GETTING CHANGING INPUTS FASTER AND FASTER SO BOTH CREW MEMBERS GETTING BUSIER AND BECOMING TIME CONSTRAINED WITH INPUTS. WE PUT IN FL290 IN RESPONSE TO A CTLR CALL REFING FL290 IN 2 MINS. PNF READ BACK CLRNC AND WE BEGAN THE DSCNT FROM FL310 TO FL290 AT A NORMAL RATE FOR THE SMALL VERT DISTANCE; PLANNING TO BE AT FL290 WITHIN THE 2 MINS MENTIONED. AT APPROX 300 FT OUT OF FL310; THE CTLR ASKED IF WE WERE STAYING AT FL310 AND PNF RESPONDED NO; WE WERE DSNDING TO FL290. AT THIS TIME THE TCASII GAVE US AN RA TO FLY UP; TURN R. ALMOST SIMULTANEOUSLY; THE CTLR SAID TO CLB TO FL310 AND TURN 40 DEGS TO THE R. I IMMEDIATELY DISENGAGED THE AUTOPLT AND COMPLIED WITH THE RA AND CTLR COMMANDS (WHICH WERE IN AGREEMENT AND WE WERE AT FL310 IN SECONDS ON THE NEW HDG. PERCEPTION WAS THAT WE WERE TO DSND TO FL290 IN 2 MINS. WHILE WE THINK WE HEARD; 'DSND TO FL290;' WHICH WE TOOK ACTION ON; BUT WAS NOT THE CASE. THE PNF DID READ BACK THE DSCNT CLRNC TO FL290 IN 2 MINS.