A B737-300 FLC FAILS TO NOTE AN UNCOMMANDED CLB DURING A DSCNT; SPD CHANGE PROC WHEN THE AUTOPLT REVERTS TO CTL WHEEL STEERING MODE. LOSS OF SEPARATION WITH TRAILING TFC OCCURS.

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OUR ACFT WAS GIVEN HOLDING AT THE ARNNE INTXN. CLRED FROM HOLDING AT ARNNE TO HOLD AT CLL; FL200. ACFT WAS SLOWED TO 225 KTS FOR HOLDING. WE ENTERED THE HOLD; NOT YET FULLY ESTABLISHED IN THE HOLD WHEN WE WERE CLRED DIRECT TO 'HOAGI' MAINTAIN 320 KTS; CROSS HOAGI AT 10000 FT AND 250 KTS. WE WERE GIVEN A R TURN TO 'HOAGI.' THE AUTOPLT WAS BEING PROGRAMMED FOR THE DSCNT. AUTOPLT WAS IN 'HDG' ROLL MODE AND PLACED IN 'LEVEL CHANGE' FOR PITCH MODE. SPD WAS IN 'MCP SPD' AND 320 KTS WAS SELECTED. THE MCP ALT WAS SET TO 10000 FT FOR THE HOAGI RESTR. AFTER DSCNT WAS STARTED FOR HOAGI; PF WAS 'HEADS DOWN' CLEANING UP THE FMC FOR CONTINUING THE CUGAR 6 ARR. PNF WAS CALLING COMPANY WITH ETA CHANGES; AND ADVISING THE FLT ATTENDANTS. NEXT THING WE SEE IS OUR ACFT CLB THROUGH FL210 WITH A TA FROM AN ACFT BEHIND US. THE AUTOPLT HAD GONE INTO A DIFFERENT MODE OF 'CTL WHEEL STEERING PITCH.' THE UNCOMMANDED CLB WAS ARRESTED AT APPROX FL212 AND A HAND FLOWN DSCNT STARTED. WE BELIEVE THAT THE MODE CHANGE WAS CAUSED BY THE LARGE PWR CHANGE FROM FLT AT 225 KTS TO 320 KTS AND THE REMOVAL OF THE 'HARD' ALT OF FL200 BEING CHANGED TO 10000 FT. ATC ADVISED US OF A 'POSSIBLE TFC CONFLICT' CAUSED BY OUR UNCOMMANDED DEV.