RPTR HAS DIFFICULTY COMPLYING WITH ARTCC INSTRUCTIONS FOR SPD AND ALT RESTRS.

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WE WERE ISSUED A DSCNT CLRNC AND CROSS 15 MI W OF MAIZE AT 11000 FT (MAIZE IIO 316 DEG RADIAL AT 30 NM). WE ACCEPTED AND ASKED IF THERE WOULD BE A SPD RESTR. ANSWER 'NO SPD RESTR.' I SET UP THE DSCNT BASED ON NORMAL SPD (.84 MACH TO 320 KIAS). AFTER THE DSCNT WAS INITIATED; ATC RECLRED US TO CROSS THE 'FIX' (15 MI W OF MAIZE) AT 280 KTS. I THOUGHT; WELL THAT'S FINE; INCREASED THE RATE OF DSCNT (AND IAS TO ABOUT 340-350 KTS). A COUPLE OF MINS LATER; CTR SAID REDUCE SPD TO 310 KTS NOW. ALSO FINE; HOWEVER WE NOW CAN'T MAKE THE RESTR. I TOLD THEM SO. THE RESPONSE WAS 'IF YOU COULD MAKE IT AT 280 KTS; WHY CAN'T YOU DO SO AT 310 KTS.' I TOLD THEM WE HAD BEEN GIVEN 280 KTS AT THE FIX NOT IN THE DSCNT. A BIT OF AN ARGUMENT TOOK PLACE. WHERE ATC DID (I THINK) ADMIT THEY WERE MISTAKEN. NEVERTHELESS; I HAD TO MAKE A MAX EFFORT DSCNT (BRAKES OUT FULLY) TO TRY TO COMPLY AS I WAS UNSURE IF THEY REALLY WAIVED THE SPD. I WOULD SUGGEST ATC NOT CHANGE ITS MIND ON DSCNT SPD ONCE THE ACFT HAS STARTED DOWN AS IT IS VERY HARD TO ADJUST ONCE YOU ARE AT SOMEWHAT OF A MAX PROFILE ALREADY.