AN MD80 FLC; IN PROX OF TSTM ACTIVITY; CLBS 1000 FT ABOVE THEIR ASSIGNED ALT WHEN THE FLT GUIDANCE COMPUTER PRODUCES SOME DISTRACTING READOUTS 30 MI E OF STL; MO.

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FLT DEPARTED THE GATE IN ST LOUIS ON MAY/TUE/00. THE FO WAS TO FLY THIS LEG. THERE WAS A DELAY IN TKOF BECAUSE OF TSTMS IN THE AREA. A REROUTE WAS ISSUED SHORTLY BEFORE TKOF AND A NEW DEP CTL FREQ WAS ISSUED. SHORTLY AFTER TKOF IT WAS DISCOVERED THAT THE AUTOPLT WOULD NOT ENGAGE. THE DEP WAS HAND FLOWN WITH THE AID OF THE FLT DIRECTOR AND AUTOTHROTTLES. THERE WAS TURB AND TSTM ACTIVITY IN THE DEP AREA AND LARGE HEADING CHANGES (IN EXCESS OF 120 DEGS) WERE ISSUED BY THE DEP CTLR FOR WX AVOIDANCE AND OTHER ACFT TFC. WE HAD BEEN ISSUED A CLRNC TO CLB TO 15000 FT AND WERE HEADED E; BUT NOT YET ON OUR ASSIGNED DEP. THE FO WAS STILL HAND FLYING THE ACFT IN THE CLOUDS IN LIGHT TO OCCASIONAL MODERATE TURB. AT APPROX 13000-14000 FT I (THE PNF) LEFT THE #1 RADIO TO USE THE #2 RADIO FOR CONTACTING THE COMPANY WITH OUR OUT AND OFF TIMES; FUEL ONBOARD AS WELL AS OUR ETA FOR ATLANTA. AS I FINISHED THIS I HEARD THE FO SAY SOMETHING TO THE EFFECT OF 'WHAT IS IT DOING; WHAT'S HAPPENING?' I LOOKED UP TO FIND THAT THE HEADING CURSOR HAD MOVED 180 DEGS AND THE FLT DIRECTOR WAS CALLING FOR A HARD R CLBING TURN; THE AIRSPD COMMAND BUG HAD MOVED TO 100 KTS AND THE AUTOTHROTTLES HAD RETARDED TO MAINTAIN ALPHA SPD. THE ALT CAPTURE WAS DISARMED AND THERE WERE NO ALT ALERT WARNINGS DURING THIS MANEUVER. BY THE TIME WE DISCONNECTED EVERYTHING AND RECOVERED CTL WE HAD PASSED THROUGH 15000 FT AND HAD STOPPED THE CLB AT 16000 FT. WE WERE AT THAT ALT FOR A VERY SHORT TIME AND STARTED A DSCNT JUST AS ATC ISSUED A CLRNC TO IMMEDIATELY DSND TO 15000 FT WHICH WE DID. IT APPEARED THAT THE FLT GUIDANCE COMPUTER WENT THROUGH SOME SORT OF INTERNAL TEST. IT APPEARED TO OPERATE NORMALLY FOR THE REMAINDER OF THE FLT. THE FLT GUIDANCE COMPUTER AND AUTOPLT WERE WRITTEN UP AS MALFUNCTIONING UPON ARR IN ATLANTA. I THINK THAT SEVERAL THINGS WERE INVOLVED IN THIS ALTDEV. I BELIEVE THAT THE FAILURE OF THE AUTOPLT TO ENGAGE WAS THE RESULT OF THE IMPENDING FLT GUIDANCE COMPUTER MALFUNCTION. THE MALFUNCTION OF THE FLT GUIDANCE COMPUTER WAS VERY CONFUSING GIVEN THE TURB AND WX CONDITIONS WHICH MADE HAND FLYING DIFFICULT AT BEST AND WAS THE CAUSE OF THIS DEV. ANOTHER PROB IS THE FACT THAT THE PNF WAS DISTRACTED BY GIVING THE REQUIRED OFF RPT TO THE COMPANY JUST AS THIS PROB WAS DEVELOPING. AUTOMATION OF THIS REQUIRED CALL WOULD HAVE HELPED GREATLY BY ALLOWING THE PNF TO MORE CLOSELY MONITOR SYS. CALLBACK CONVERSATION WITH RPTR REVEALED THE FOLLOWING INFO: THE CAPT STATED THAT THIS WAS AN EARLIER MODEL OF THE MD80; PURCHASED FROM A FOREIGN AIRLINE. THE THROTTLES HAD RETARDED TO THE MINIMUM CTL SPD FOR THE ACFT. THE CONTRACT MAINT PERSONNEL AT ATL COULD NOT IMMEDIATELY DEFINE THE PROB SO IT IS NOT KNOWN WHAT THE SIGN OFF WAS. THEY DID RESET ALL OF THE FLT GUIDANCE COMPUTERS. CAPT WAS ASKED TO CALL ATC WHEN ARRIVING AT DEST ARPT. CAPT RECEIVED A CHK RIDE AND THE FO RECEIVED A SIMULATOR REVIEW RIDE. SUPPLEMENTAL INFO FROM ACN 472377: WE WERE CLEARED TO 15000 FT AND WERE 20 TO 30 MILES ESE OF STL VOR HEADING E. HDG BUG MOVED TO A HDG OF ABOUT 285 DEGS. AUTOTHROTTLES RETARDED TO MAINTAIN SELECTED SPD 100 KTS. ALT CAPTURE WAS DISARMED AND NO ALT WARNINGS WERE HEARD.