B737-700 FLT CREW RECEIVES STICK SHAKER WHEN PWR WAS NOT RESTORED FOR LEVEL FLT AFTER A CRUISE DSCNT.

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FLT WAS APPROX 275 MI FROM LAS AT FL400; WHEN THE FO NEEDED TO GO TO THE RESTROOM. WE WENT THROUGH THE APPROPRIATE STEPS AND AFTER ABOUT A 10 MIN WAIT; THE FLT ATTENDANT TOLD US THAT IT WAS CLR TO COME OUT. PRIOR TO HIS DEP; ATC HAD ISSUED US A DSCNT CLRNC TO FL360. THE FO WAS THE PF AND HE INITIATED A CRUISE DSCNT USING VERT SPD AT 1000 FPM AND REDUCING PWR TO MAINTAIN AIRSPD. HE DEPARTED THE FLT DECK WHEN THE ACFT WAS APPROX OUT OF FL380. BEFORE HE DEPARTED; THE FO XFERRED CTL OF THE ACFT AND I ACKNOWLEDGED SAME. HE DEPARTED AND THE FLT ATTENDANT ENTERED THE FLT DECK. THE ACFT LEVELED AT FL360 BUT I FAILED TO NOTICE THE LEVELOFF. I'M SURE THAT MY ATTN WAS DIRECTED SOMEWHAT AFT; TO THE FLT ATTENDANT. AFTER A FEW MINS; I NOTICED THAT THE AIRSPD HAD DECAYED. I ADVANCED THE T/L AND REQUESTED A LOWER ALT. THE STICK SHAKER ACTIVATED. THE CTLR CLRED ME TO FL320 AND I DISENGAGED THE AUTOPLT AND STARTED A DSCNT TO FL320. THE ACFT RECOVERED RELATIVELY QUICKLY AND THE FO AND THE FLT ATTENDANT EXCHANGED POS AS THE ACFT WAS OUT OF FL330 FOR FL320. THE ACFT PROCEEDED TO LAS WITHOUT FURTHER INCIDENT. CONTRIBUTING FACTORS: 1) CRUISE DSCNT; DONE FOR FUEL CONSERVATION; FEELS AND SOUNDS LIKE CRUISE; IDLE DSCNTS WOULD NOT. 2) XFERRING CTL AND THEN LEAVING THE FLT DECK DURING A DSCNT THAT THE PF (VACATING) INITIATED. 3) THE REQUIREMENT OF THE FLT ATTENDANT TO ENTER THE COCKPIT. HE/SHE MAY BE VIEWED AS A POSSIBLE DISTR. MY FEELINGS ARE SPLIT BTWN: 1) I CAN'T BELIEVE IT HAPPENED; AND 2) I CAN'T BELIEVE IT NEVER HAPPENED TO ME BEFORE. IT WAS THAT INSIDIOUS. WHAT CAN BE DONE TO PREVENT IT: 1) AVOID BATHROOM BREAKS DURING DSCNTS. 2) COUPLE THE XFER OF CTL WITH A SHORT BRIEF; EG; 'THE ACFT IS DSNDING; 2000 FT TO GO TO LEVELOFF.' SUPPLEMENTAL INFO FROM ACN 695150: WE DISCUSSED THE EVENT; AND THIS IS WHAT I PERCEIVED TO HAVE OCCURRED: I LEFT COCKPIT IN DSCNT WITH SOME PWR SET. FLT ATTENDANT ENTERED; ENGAGED THE CAPT IN CONVERSATION WHICH DISTR HIS PRIMARY ATTN AWAY FROM THE INSTS. THE ACFT DSNDED AND LEVELED OFF AT FL360 AS COMMANDED; BUT FULL CRUISE PWR WAS NOT RE-ESTABLISHED. ACFT CONTINUED TO SLOW WHILE LEVEL UNTIL STICK SHAKER OCCURRED. CAPT RECOGNIZED THIS; APPLIED FULL THRUST TO NO AVAIL; HAD PRESENCE OF MIND TO SEE THAT THERE WAS AN ACFT BELOW US AT UNDETERMINED DISTANCE; REQUESTED A DSCNT FROM CTR; WHICH WAS IMMEDIATELY GRANTED TO FL320. NO TCAS ALERT OCCURRED; NO COMMENT FROM CTR; NO PERCEIVED NEED TO DECLARE EMER (WHICH I AGREE WITH). NOSE WAS SMOOTHLY LOWERED AND AIRSPD RAPIDLY RESTORED. NEITHER FLT ATTENDANTS NOR PAX WERE ALARMED BY THIS INCIDENT. I MADE A PA TO PAX TO EXPLAIN DSCNT AND VIBRATION; EXPLAINED THAT WE HAD ENCOUNTERED TURB AND MOUNTAIN WAVE ACTIVITY; WHICH WE TRULY HAD XING ROCKIES AND SUBSEQUENTLY ENCOUNTERED FURTHER ON OUR RTE AND DSCNT INTO LAS. ANALYSIS: SEVERAL CONTRIBUTING FACTORS INCLUDED DSCNT; XFER OF CTLS WHILE IN DSCNT; STRONG DISTR OF FLT ATTENDANT IN COCKPIT WANTING TO TALK AND GET A BREAK FROM BUSY ACTIVITIES IN CABIN. FOR MY PERSONAL KNOWLEDGE AND EXPERIENCE GAIN FROM THIS INCIDENT; I WILL ENDEAVOR NOT TO XFER CTLS IF NOT IN STABILIZED; LEVEL FLT. IF I HAVE TO DO SO; I WILL FULLY BRIEF PHASE OF FLT (CLB OR DSCNT; ETC); PWR SET OR NOT; WHAT ALT CLRED TO NEXT; ETC; BEFORE XFERRING CTLS. THIS EVENT ALSO ILLUSTRATES IN VIVID DETAIL HOW DISTRACTING FLT ATTENDANTS CAN BE IN COCKPIT (THROUGH NO FAULT OF THEIR OWN); AND HOW WE MUST STRICTLY CONCENTRATE ON MAINTAINING FOCUS AND SITUATIONAL AWARENESS ON THE MISSION AT HAND; NAMELY FLYING THE AIRPLANE FIRST BEFORE ALL ELSE. AVIATE; NAV; COMMUNICATE!