ALTDEV ALT EXCURSION ON TCASII RA. POTENTIAL CONFLICT LTSS.

Narrative

AS WE ARRIVED INTO ATL WE WERE GIVEN A DSCNT TO 11000. SOON AFTER; ATC CALLED TFC AT 12 O'CLOCK FOR WHICH WE COULD NOT SEE DUE TO IMC. WITHOUT ANY KIND OF TA THE TCASII SYS TOLD US TO 'CLB; CLB.' THE AUTOPLT WAS DISCONNECTED AND A CLB WAS ESTABLISHED. THE TCASII ISSUED ANOTHER ADVISORY TO 'MONITOR VERT SPD' AND OUR CLB WAS DECREASED. AT ABOUT THIS TIME LIGHTS FROM AN ACFT DIRECTLY BELOW US COULD BE SEEN THROUGH THE CLOUDS. APCH CTL RADAR WAS VERY BUSY AND A CALL TO TELL THEM WE HAD ENCOUNTERED COULD NOT BE WORKED IN QUITE YET. THEY SOON SAW A DEV AND CONTACTED US TO SAY THAT 11000 FT WAS ASSIGNED AND THAT WE HAD TFC 12 O'CLOCK 5 MI. THE CONFLICT ACFT WAS RPTED BY ATC TO BE ASSIGNED 10000 FT. AT ABOUT THIS TIME A 'CLR OF CONFLICT' MESSAGE WAS RECEIVED AND WE RETURNED TO A DSCNT DOWN TO 11000. (ESTIMATE THAT A CLB WAS TOPPED AT 11700-11800 FT). WE RETURNED TO 11000 FT; BROKE INTO VMC AND RECEIVED A 'TA' FROM THE TCASII NOW FOR THE 12000 FT TFC WHICH WE PICKED UP VISUALLY. WHAT PROBABLY HAPPENED HERE IS THAT WE WERE DSNDING THE OTHER ACFT WAS CLBING AND THE TCASII ALERTED ON THE POTENTIAL CONFLICT NOT KNOWING OF THE ASSIGNED LEVEL OFFS. IT MIGHT HELP TO KNOW THAT OUR TFC IS CLBING TO LEVEL AT 10000 OR DSNDING TO LEVEL AT 11000. THIS INCIDENT REINFORCED MY BELIEF THAT TCASII IS A GOOD SYS; IT MUST BE BELIEVED AND ACTION TAKEN.