AJ25. FO LOST CTL WHEN AFCS TRIPPED OFF WITHOUT FO'S KNOWLEDGE.

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ACFT WAS BEING FERRIED TO AGS FOR A PRE-BUY INSPECTION. NO PAX ONBOARD. AFTER SETTING ACFT IN CRUISE CONFIGN AT FL370 I TURNED THE ACFT OVER TO THE R SEAT PLT SO THAT I COULD GET THE BREAKFAST IN THE CABIN'S GALLEY. WHILE THERE I DECIDED TO CHK OUT THE LAVATORY LIGHT THAT WAS CONTINUALLY ON IN THE COCKPIT. UNABLE TO EASILY FIND THE CAUSE OF THE LIGHT; I RETURNED TO EAT MY BREAKFAST. ABOUT 5 MINS INTO THE MEAL; THE ACFT SUDDENLY BEGAN A RAPID DSNDING R TURN. ENTERING THE COCKPIT I FOUND THE FO FIGHTING TO RESTORE THE ACFT ATTITUDE AND RETURN TO FL370. IN HIS EFFORT TO DO SO HE OVERCORRECTED THE CTLS TO ALLOW THE ACFT TO CLB TO FL380. STRAIGHT AND LEVEL FLT AT FL370 WAS THEN RESUMED WITHOUT FURTHER INCIDENT. ATC DID NOT QUESTION THE EXCURSIONS OF OUR ALT (FROM FL370 TO FL360 TO FL380 BACK TO FL370). WHEN THE ACFT WAS BACK TO NORMAL FLT I QUESTIONED THE COPLT FOR THE REASON. HIS RESPONSE WAS THAT THE AUTOPLT HAD TRIPPED OFF WITHOUT HIM HEARING IT. HIS ATTN WAS ON HIS CHART REVIEWING OUR RTE OF FLT AND DID NOT NOTICE THAT THE ACFT WAS ENTERING A SPIRAL TO THE R UNTIL THE ALT ALERTER WENT OFF. BY THAT TIME THE DSCNT AND BANK HAD RAPIDLY INCREASED TO WHERE CTL WAS DIFFICULT WITHOUT OVERSTRESSING THE PLANE. FORTUNATELY THERE WEREN'T ANY OTHER ACFT IN THE IMMEDIATE VICINITY AND NO CONFLICTS OCCURRED. THIS OCCURRENCE COULD HAVE BEEN AVOIDED IF THE PLT AT THE CTLS MAINTAINS A CONTINUOUS MONITORING OF THE AUTOPLT AND FLT CONDITIONS WHILE THE OTHER PLT IS NOT AT THEIR STATION.