ALTDEV ALT EXCURSION FROM ASSIGNED. TEMPORARY LOSS OF ACFT CTL DURING RADAR VECTORING AND ATTEMPT TO CLB BACK TO ALT.

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WHILE CRUISING AT FL370 SFO-LAX; I WAS TALKING TO THE FLT ATTENDANTS ON THE INTERPHONE AND WAS NOT ACTIVELY MONITORING THE VHF FREQ ALTHOUGH I HAD IT ON AT A LOWER VOLUME. THE CAPT WAS TALKING TO ATC ABOUT SOME TURB AND I DID OVERHEAR THE MENTION OF FL350 AS I ENDED MY COM WITH THE FLT ATTENDANT. AT THAT TIME THE CAPT SELECTED 35000 FT IN THE ALT WINDOW. I CONFIRMED THAT 35000 FT WAS SET IN THE WINDOW IN ACCORDANCE WITH SOP; BUT SINCE I WAS NOT ACTIVELY MONITORING ATC AT THAT TIME (I WAS AS SOON AS I ENDED MY COMS WITH THE FLT ATTENDANT) I ADVISED ATC THAT WE WERE LEAVING FL370 FOR FL350 AND THE CTLR ROGERED MY COM. LEAVING 36300 FT THE CTLR INSTRUCTED US TO MAKE AN IMMEDIATE 40 DEG L TURN AND MAINTAIN FL370 DUE TO TFC AT FL350 (TO WHOM HE ALSO GAVE A 40 DEG TURN). I ACKNOWLEDGED AND ADVISED HIM THAT WE WERE CLBING TO FL370. WHEN THE CTLR INSTRUCTED US TO MAKE A 40 DEG L TURN; THE CAPT DISENGAGED THE AUTOPLT AND ROLLED INTO A 45 DEG ANGLE OF BANK AND ATTEMPTED TO START A CLB WHICH RESULTED IN MODERATE BUFFET (ACCELERATED STALL). I STATED LOUDLY 'DON'T STALL' AND HE RELEASED THE BACK PRESSURE AND WE RECOVERED. WE ROLLED OUT ON THE ASSIGNED HDG AND CLBED TO FL370. I DON'T NOW IF WE WERE EVEN CLRED TO FL350 AND WHEN I LATER STATED TO THE CTLR THAT I ADVISED HIM WE WERE LEAVING FL370 FOR FL350; HE SAID HE WAS WORKING 2 FREQS. THE REMAINDER OF THE FLT WAS UNEVENTFUL. COMMENTS: THE CAPT OVERREACTED. A 45 DEG BANK AT 36000 FT WAS EXCESSIVE. DON'T CREATE AN EMER TO AVOID A POTENTIAL EMER. WHEN IN DOUBT; ASK FOR A CONFIRMATION OF CLRNC. TO ENDANGER THE ACFT TO AVOID BREAKING THE 5 MI BUBBLE WAS UNWISE (AT NO TIME DID WE RECEIVE A TA OR RA ON THE TCASII).