A G1159 IN CRUISE AT FL290 AND WHILE SWITCHING THE AUTOPLT'S CHANNELS TO CHK A SLIGHT L AND R ROLL; THE AIRPLANE SUDDENLY PITCHED UP. HELD HVY NOSE DOWN COLUMN PRESSURE AND 2 UNITS OF MANUAL NOSE DOWN TRIM TO CORRECT CLB.

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I DEPARTED LAS FOR ZZZ VIA THE ZZZZZZ SID THEN WAS CLRED DIRECT ZZZ VOR. I ALSO CLBED TO MY ASSIGNED ALT OF FL290. I WAS CRUISING IN SMOOTH AIR I NOTICE A SLIGHT ROLL L AND R. THE AUTOPLT WAS ENGAGED. I SELECTED THE AUTOPLT OFF AND DESELECTED THE #1 ROLL CHANNEL AND TURNED AUTOPLT BACK ON; THE ROLL L AND R WAS STILL THERE; SO I TURNED AUTOPLT OFF AND DESELECTED THE #2 ROLL CHANNEL THEN I SELECTED THE AUTOPLT ON; ROLL L AND R CONTINUED (YOU CANNOT DESELECT BOTH ROLL CHANNELS). SO I DESELECTED AUTOPLT AND SELECTED IT BACK ON. THE AUTOPLT SWITCH WAS FROZEN TO THE OFF POS. I OBSERVE NO TRIM OR ROLL CAUTION LIGHTS. I LOOKED DOWN AT THE AUTOPLT SWITCH AND MADE SURE THE MANUAL ROLL SWITCH WAS IN THE DETENT. IF IT'S NOT THIS WILL KEEP YOU FROM TURNING THE AUTOPLT ON. WHILE I WAS DOING THIS THE AIRPLANE BEGAN A RAPID CLB. I APPLIED FORWARD YOKE PRESSURE. I WAS ONLY ABLE TO REDUCE THE RATE. I ASK THE COPLT TO TRIM SWITCH. I HAVE NEVER HAD TO PUSH FORWARD LIKE THAT. AFTER COPLT SHUT OFF TRIM I MANUALLY MOVED THE TRIM DOWN BUT NOT BY MORE THAN 2 DEGS TO 2 1/2 DEGS. AT THAT POINT ACFT PITCH FELT NORMAL; I DSNDED BACK DOWN TO FL290. WE CLBED TO ABOUT FL298 OR 299 BEFORE I HAD NORMAL PITCH COMMAND. ZZZ CTR ASK US ABOUT OUR ALTDEV. COPLT SAID WE WERE CORRECTING AND THAT WE HAD A RUNAWAY TRIM FOR A COUPLE SECONDS. AFTER THE EVENT WE CONTINUED TO ZZZ. WE INFORMED OUR MAINT DIRECTOR. THE AIRPLANE WAS GNDED. WE HAVE A MECH OF OURS THAT WAS ONBOARD. WE HAVE AN AVIONICS MAN HERE IN ZZZ. THEY HAVE CONTACTED GULFSTREAM AND HONEYWELL REPRESENTATIVES FOR HELP IN TROUBLESHOOTING. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE AIRPLANE WITH THE AUTOPLT ENGAGED DISPLAYED A SLIGHT L AND R ROLL NOT MUCH BUT NOTICEABLE. THE L AND R AUTOPLT CHANNELS WERE DESELECTED ONE AT A TIME TO ISOLATE THE FAULTY CHANNEL. DURING THIS SWITCHING OF THE AUTOPLT ROLL CHANNELS; THE PITCH CHANNEL BEGAN A RAPID CLB AND ONLY WITH HVY CTL COLUMN FORCE AND MANUAL PITCH TRIM GREATER THAN 2 1/2 UNITS WAS THE CLB ARRESTED. ATC CONTACT WAS MADE AND WERE ADVISED OF RUNAWAY AUTOPLT TRIM PROB AND CORRECTION. ON THE GND; LCL AVIONICS TECHNICIANS WERE UNABLE TO FIND ANY FAULTS DURING TESTING AND THE AIRPLANE AND AUTOPLT MANUFACTURERS WERE CALLED OUT AND RESULTED IN SEVERAL UNITS BEING REPLACED; DUAL CHANNEL AUTOPLT CTL UNIT; PITCH TRIM SERVO; AND STABILIZER AUGMENTATION UNIT. AFTER REPLACEMENT OF THESE UNITS THE AIRPLANE HAS FLOWN WITH NO TRIM OR AUTOPLT RPTS. THE REMOVED UNITS ARE STILL IN TESTING AND NO RESULTS HAVE BEEN RPTED.