CRJ FLT CREW EXPERIENCES MOMENTARY LOSS OF ACFT CONTROL WHEN AUTOPILOT DISCONNECTS DUE TO WINDSHEAR. ALTDEV RESULTS.

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THE FO WAS FLYING AUTOPLT ON. WE WERE 17 NM OUT OF ATL ON A DOGLEG TO FINAL FOR THE ILS RWY 27L. A POWERFUL FRONT HAD JUST MOVED THROUGH; THE SAME ONE THAT SPAWNED TORNADOES. THE BULK OF A SQUALL LINE WAS SEVERAL MI E OF US. THERE WERE LIGHT TO MODERATE RAIN SHOWERS THAT ATL APCH WAS VECTORING US THROUGH. WE HAD EXPERIENCED OCCASIONAL LIGHT TO MODERATE TURB FROM ABOUT 30 NM OUT. JUST BEFORE LOC INTERCEPT ATC ASSIGNED A SPD OF 190 KIAS. WE PUT THE FLAPS TO 8 DEGS TO SLOW FROM 210 KIAS. THRUST LEVERS WERE ALL THE WAY BACK AT IDLE BUT THE ACFT DID NOT APPEAR TO BE SLOWING DOWN AT A NORMAL RATE. IN ANTICIPATION OF POSSIBLE WINDSHEAR; I PROMPTED THE FO TO KEEP THE ENGS SPOOLED TO 45% N1 MINIMUM. JUST AS THE ENGS SPOOLED UP; A VERY UNIQUE EVENT OCCURRED WHICH TRIGGERED A STALL WARNING AND AUTOPLT MALFUNCTION. WE ENCOUNTERED AN UPDRAFT OF HIGH INTENSITY BUT EXTREMELY SHORT DURATION; NOT EVEN 1/4 SECOND. THE IMPULSE WAS SO SHORT THAT NO APPRECIABLE CHANGE IN ACFT ALT OR ATTITUDE OCCURRED. HOWEVER; SINCE THIS UPDRAFT BROUGHT WITH IT AN INHERENT INCREASE IN ANGLE OF ATTACK; THE ACFT SAW FIT TO ACTIVATE THE STICK SHAKER FOR A SPLIT SECOND WHICH RESULTED IN AN AUTOPLT DISCONNECT. THIS ALL OCCURRED AT THE SAME TIME WE HAD TO MAKE THE TURN TO INTERCEPT FINAL AND ANSWER A CALL FROM ATC. WHEN THE FO GOT CTL OF THE ACFT AGAIN THE ACFT HAD DRIFTED 300 FT BELOW THE ASSIGNED ALT OF 7000 FT. THIS RESULTED SLOWLY DUE TO THE NOSE DOWN TRIM CONDITION AFTER AUTOPLT FAILURE NOT DURING THE ACTUAL 'STALL.' THE NOSE DOWN TRIM CONDITION OBVIOUSLY OCCURRED BECAUSE OF THE RAPID REDUCTION IN AIRSPD ONCE THE LOW PWR SETTING TOOK EFFECT. THIS EVENT WAS RELATIVELY INCONSEQUENTIAL; HOWEVER; WHAT IF WE HAD BEEN CLOSER TO THE GND AND THIS TIME THE UPDRAFT WAS STRONG ENOUGH TO TRIGGER A STICK PUSHER? THAT COULD HAVE BEEN MUCH MORE SERIOUS. AUTOMATION CAN DO SOME STRANGE THINGS SOMETIMES.