CONFLICT OCCURRED 20 NM FROM SCK BTWN A DSNDING VFR ROCKWELL TURBO COMMANDER AC680W AND XING DC10 ON FEW WITH ZOA.

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WE WERE MAINTAINING 17500 FT VFR IN STOCKTON AREA PRIOR TO DSNDING TO LAND AT STOCKTON. ZOA REQUESTED CALL PRIOR TO DSCNT (VFR FLT FOLLOWING). 2 CALLS MADE TO CTR -- NO RESPONSE. NO OTHER TFC HEARD ON FREQ. COMMENCED DSCNT. OTHER TFC HEARD ON FREQ. MY RADIO OPERATING OK. CONTINUED CALLING ZOA. ZOA RESPONDED TO 2 CALLS WITH MY CALL SIGN AND SAYING MY CALL BLOCKED BUT NO OTHER BLOCKING TFC HEARD. REPEATED CALL AND ZOA RESPONDED WITH A DSCNT RESTR WHICH WAS NOT HEARD AS THE LEVEL WAS TOTALLY CUT OFF FROM END OF XMISSION. WE WERE AT APPROX 14000 FT AND DSNDING. AN IMMEDIATE CALL TO ZOA TO REPEAT THE LEVEL WAS MADE AND THEN RECEIVED. AS THIS CALL WAS BEING MADE; OTHER 2 CREW MEMBERS SAW DC10 PASSING BELOW AND BEHIND US AT POSSIBLY LESS THAN 1000 FT SEPARATION. WE WERE THEN LEVELING AT 13500 FT AND THEN CLBED BACK IMMEDIATELY TO 14000 FT WHEN THIS LEVEL WAS RECEIVED FROM ZOA. I TOLD ZOA WE HAD BEEN CALLING REPEATEDLY AND THAT THEIR XMISSIONS WERE BREAKING UP. WE WERE XFERRED TO NORCAL DEP. AT THAT TIME; ASKED FOR A RADIO CHK WHICH WAS PERFECT AND WHEN QUERIED BY THEM ADVISED WE HAD SEEN THE DC10 VISUALLY AND THAT IT HAD PASSED BELOW AND BEHIND. NO MORE WAS HEARD FROM ATC ABOUT THIS AND I DO NOT KNOW IF THEY CONSIDERED A SEPARATION BREAKDOWN TO HAVE OCCURRED. (WE WERE BOTH ON RADAR AT THE TIME.) UNABLE TO EXPLAIN ZOA'S BEING UNABLE TO HEAR MY INITIAL CALLS; MY BLOCKED CALLS AND THEIR RESPONSES BEING BROKEN. ACFT RADIO EQUIP WAS WORKING PERFECTLY AND HAS NO HISTORY OF XMISSION OR RECEIVER PROBS.