CAPT OF AN SMT EXECUTIVE JET MOMENTARILY LOST CTL OF THE ACFT WHEN THE AUTOPLT PITCHED THE NOSE UP IN A CLB. THE RPTR DISCONNECTED THE AUTOPLT AND PITCH TRIM AND ROLLED THE ACFT FOR A NOSE DOWN POS IN ORDER TO GET BACK TO ASSIGNED ALT. ATC NOTICED THE ALT EXCURSION.

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ON THE AUTOPLT AT FL410 WE WERE CLRED DOWN TO FL350. THE DSCNT WAS STARTED ON THE AUTOPLT. ZID CALLED BACK AND SAID I COULD MAINTAIN FL410. THE CTLR THOUGHT WE WERE LNDG IN OHIO; THEN REALIZED OUR DEST WAS IN PENNSYLVANIA. WE WERE AT FL405 AT THAT TIME SO I USED THE CTL WHEEL TO REVERSE THE DSCNT AND CLB BACK TO FL410. UPON REACHING FL410; THE AUTOPLT WAS CHASING THE ALT AND NOT SMOOTH LIKE BEFORE THE DSCNT. I WAS HAND FLYING TO KEEP THINGS SMOOTH. ZOB STARTED US DOWN A LITTLE LATER. FL350; FL330; AND THEN FL290 AND PLEASE EXPEDITE. WE WERE COMING DOWN AT ABOUT 3000 FPM. CTR ASKED IF I COULD BE LEVEL IN 70 SECONDS. I REPLIED THAT IT WOULD BE NO PROB. I CALLED LEVEL AS REQUESTED AND PUT THE AUTOPLT ON WITH ALT HOLD. I NOTICED NOTHING UNUSUAL AND WAS ADDING PWR FOR NORMAL CRUISE. THE ACFT PITCHED UP SHARPLY AND THE FLT DIRECTOR WAS COMMANDING 10 DEGS UP. I USED THE AUTOPLT PITCH TRIM WHEEL TO COMMAND DOWN PITCH. THE PITCH CONTINUED UP IN SPITE OF MY DOWN COMMAND. I THEN THOUGHT I HAD RUNAWAY PITCH TRIM. I HIT THE DISCONNECT (TRIM AND AUTOPLT); ROLLED THE ACFT TO 30-35 DEGS BANK AS I PUSHED THE NOSE DOWN. STILL TRYING NOT TO DISTURB THE PAX. THE DEV WAS IMMEDIATE (ALMOST 700 FT HIGH) AND THE CORRECTION WAS FAST BACK TO FL290. ATC AND I TALKED ABOUT WHAT HAD HAPPENED AND I WAS GIVEN A PHONE NUMBER TO CALL. DURING FURTHER FLT AND DSCNTS; I TRIED THE AUTOPLT AND IT TRIED TO GO TO 10 DEGS PITCH UP. DURING THESE ATTEMPTS; I WAS AWARE OF THE PROB AND DID NOT LET IT GET AWAY. AFTER LNDG; THE AUTOPLT WOULD NOT TEST PROPERLY. AFTER FURTHER INVESTIGATION AND CTRING OF THE AUTOPLT PITCH CTL WHEEL; THE AUTOPLT TESTED OK. ON TEST FLT NEXT DAY ALL SEEMED OK. AVIONICS WILL CHK IT OUT. I CALLED ZOB AS REQUESTED AND THE NUMBER NEVER ANSWERED. I THEN CALLED FSS AND GOT ANOTHER NUMBER WHICH DID ANSWER AND AFTER TALKING TO A COUPLE OF DIFFERENT PEOPLE; I WAS TOLD TO CALL ANOTHER NUMBER MONDAY MORNING.