B757 FLT CREW EXPERIENCED CONFUSION AS TO WHAT ALT WAS ISSUED BY ZID CTLR DURING BUSY TFC PERIOD.

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WE WERE LEVEL AT FL250; SPD REDUCED TO 280 KIAS WITH ZID. MY COPLT ASKED ZID IF WE COULD HAVE NORMAL SPD. ZID REPLIED; 'YOU HAVE TFC XING AHEAD; CLB TO FL260.' COPLT REPLIED; AND SAID 'WHAT DID THAT HAVE TO DO WITH THE SPD?' WE SET FL260 AND CLBED. AT ABOUT LEVELOFF; ZID SAID; 'ACR X; CONFIRM ALT AS FL250.' COPLT REPLIED; 'WHAT?' CTR PROMPTLY GOT INTO A LONG CONVERSATION WITH ANOTHER ACFT. THINKING THAT MAYBE HE MEANT TO SAY XING ACFT AT FL260; I RETURNED TO FL250. WE SPENT THE NEXT 4 OR 5 MINS TRYING TO GET ZID TO CONFIRM WHAT ALT HE WANTED US AT. BTWN HIS CONSTANT TALKING TO OTHER ACFT; HAVING OTHER ACFT STEP ON OUR XMISSIONS AND CTR BEING OFF FREQ AND NOT ANSWERING; THIS WAS A VERY TENSE TIME FOR ME! ZID EVENTUALLY SAID 'MAINTAIN FL250.' HE SAID NOTHING ELSE ABOUT THE INCIDENT AND AGAIN STARTED A BUSY CONVERSATION WITH ANOTHER ACFT. WE THEN GOT A FREQ CHANGE AND NEVER GOT TO DISCUSS IT WITH THE CTLR. A VERY DISTURBING DAY! THIS IS UNFORTUNATELY COMMON; WHERE CTR IS TOO BUSY TO TALK TO US AND CORRECT A PROB. AS FAR AS I KNOW; NO CONFLICT. NO TCAS ALERT.