THE GAR PROC AS USED BY A B737-800 PIC CREATES AN UNUSUALLY HIGH PITCH ANGLE; PRODUCING AN APCH TO A STALL WHEN ENG THRUST WAS REDUCED ON THE SHORT CLB TO LEVEL OFF 5 MI W OF ATL; GA.

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ACFT WAS IN THE FINAL APCH PHASE OF THE APCH TO RWY 8L IN ATL. WE HAD BEEN RADAR VECTORED TO A 17 MI FINAL AT 5000 FT AND 180 KTS. WE WERE THEN ASSIGNED 160 KTS AND CLRED FOR THE ILS RWY 8L APCH. THE 'APCH' MODE WAS SELECTED WITH A SINGLE AUTOPLT ENGAGED AND THE ACFT INTERCEPTED AND TRACKED THE ILS NORMALLY. THE ACFT WAS CONFIGURED FOR A FLAPS 30 DEG LNDG AND NORMAL CHKLISTS WERE ACCOMPLISHED. THE ACFT WAS STABILIZED ON APCH AND SPACING WITH OTHER TFC APPEARED TO BE COMFORTABLY SPACED ON TCASII. XING THE FAF AT 2800 FT; THE TWR CTLR ISSUED A CLRNC TO CLB TO 4000 FT AND TO TURN L TO 360 DEGS. I DID NOT HEAR CLRLY THE CALL SIGN ON THE CLRNC. I LOOKED TO THE FO AND ASKED HIM TO VERIFY THE CLRNC BEING FOR US. MY HANDS WERE ON THE FLT CTLS AS I WAS 'FOLLOWING' THE AUTOPLT ON THE APCH. AS THE FO VERIFIED THE CLRNC; I SELECTED 'TOGA' MODE OF FLT AUTOMATION AND PROCEEDED WITH THE NORMAL GAR CALLOUTS. SELECTING TOGA AUTOMATICALLY DISCONNECTED THE AUTOPLT AND ESTABLISHED NEARLY FULL PWR ON BOTH ENGS. AS I WAS NOT LOOKING DIRECTLY AT THE FLT INSTS WHEN SELECTING TOGA; THE VERY RAPID INCREASE OF PWR CAUSED THE ACFT PITCH TO INCREASE PAST THE DESIRED ATTITUDE OF 15 DEGS TO AN ATTITUDE OF 20 DEGS; OR POSSIBLY SLIGHTLY HIGHER. ALTHOUGH I INSTINCTIVELY PLACED FORWARD PRESSURE ON THE FLT CTLS TO COUNTER THE RAPID CHANGE IN PITCH; THE PRESSURE WAS INSUFFICIENT TO STOP THE PITCH AT THE DESIRED ATTITUDE. IN AN ATTEMPT TO SMOOTHLY LOWER THE NOSE IN THE INTEREST OF PAX COMFORT; THE ACFT EXPERIENCED A 1 OR 2 SECOND STICK SHAKER WARNING AS WE LEVELED AT 4000 FT. CONTRIBUTING FACTORS: 1) AN UNEXPECTED CONDITION: AN UNEXPECTED GAR AT AN UNEXPECTED PHASE OF FLT; 2) AUTOMATION WHICH CONTRIBUTES TO A LARGE 'SURPRISE FACTOR: LARGE AND RAPID PWR CHANGE IN ENGS WELL BELOW THE WING CREATING AN INSTANT PITCH CHANGE; AND THEN DISCONNECTING THE AUTOPLT. 3) THE SELECTION OF TOGA AT A TIME WHEN CONCENTRATION WAS NOT FIRMLY ESTABLISHED ON FLT INSTS.