ON CLB THE PERFORMANCE MGMNT SYS DEFAULTED TO THE AUTOPLT VERT SPD LOGIC WHICH REQUIRED THE GIVEN VERT SPD AT THE EXPENSE OF AIRSPD. THE CREW DID NOT NOTICE THE SPD DETERIORATION UNTIL ACFT COMMENCED VIBRATION AT THE BEGINNING OF THE STALL.

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THIS EVENT OCCURRED DURING CLBOUT WITH EXCELLENT WX CONDITIONS. THE AUTOPLT HAD BEEN ENGAGED AT 15000 FT WITH PERF AND NAV MODE SELECTED. IT APPEARED TO BE FUNCTIONING WELL. NOT MUCH WAS OCCURRING IN THE COCKPIT; AND THE CAPT BEGAN HIS PA'S TO THE PAX. I OPENED MY CHARTS UP TO REVIEW DEST APCHS; AS IT HAD BEEN A WHILE SINCE I'D BEEN TO PDX. I FELT A VIBRATION; LOOKED UP; AND NOTICED MY AIRSPD HAD DETERIORATED AND THE FMA INDICATED 'SPD LOW.' I IMMEDIATELY CLICKED OFF THE AUTOPLT; VERIFIED MLT PWR; AND LEVELED THE WINGS; I ASKED THE CAPT TO ASK ATC FOR A DSCNT AND IT WAS APPROVED. I SLOWLY REBUILT AIRSPD; AND WITH CTR APPROVAL; RESUMED THE CLBOUT. THE ACFT NEVER STALLED; BUT IT WAS LITERALLY ONLY A FEW SECONDS/KTS FROM DOING SO. SOMEHOW; THE PERF MODE HAD DESELECTED DURING THE CLB; AND IT FAILS INTO THE VERT SPD REGIME. THE CLB RATE THE AUTOPLT HAD WHEN PERF DESELECTED NEARLY RESULTED IN A STALL; AS THE ACFT BLED ITS SPD TO MAINTAIN THE CLB. MY FAILURE TO MAINTAIN AN ADEQUATE SCAN WAS THE PRIMARY CAUSE OF THIS NEAR STALL INCIDENT. I RELIED TOO MUCH ON THE AUTOPLT; AND ALLOWED MYSELF TO BECOME DISTRACTED WITH MY CHART REVIEW. THAT SHOULD HAVE BEEN DONE AT CRUISE; WITH THE CAPT 'COVERING' FOR ME WHILE I HAD MY HEAD IN THE BOOKS. ALSO; THE PNF MIGHT HAVE NOTICED LOW SPD SOONER IF HE'D MADE HIS PA AT LEVEL OFF; NOT IN THE CLB. I'LL NEVER UNDERESTIMATE THE POTENTIAL FOR DISASTER THAT OVER-RELIANCE ON AN AUTOPLT HOLDS AGAIN.