B737-300 CREW DID NOT COMPLY WITH A XING RESTR ALT BECAUSE OF IMPROPER ACFT FMC OP IN ZDC CLASS E AIRSPACE.

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WE WERE GIVEN A XING RESTR 'SOMTO AT 13000 FT.' WE HAD BEGUN A VERT SPD DSCNT DUE TO LIGHT CHOP; THUS DISCONNECTING THE VNAV FUNCTION OF OUR FMC. OUR RATE OF DSCNT WAS NOT SUFFICIENT TO MAKE THE 13000 FT RESTR. ATC HAD US CONFIRM OUR ALT OVER SOMTO -- 17000 FT. THEY REQUESTED THAT WE THEN EXPEDITE DOWN TO 11000 FT; WHICH WE DID. WHEN WE QUERIED THEM OF ANY POTENTIAL HAZARD/PROB; THEY RESPONDED THAT THERE WAS NO CONFLICT. AVOIDING ANOTHER SUCH OCCURRENCE WILL INVOLVE A BETTER MONITORING OF THE ACFT'S AUTOMATION. SUPPLEMENTAL INFO FROM ACN 504200: ON DSCNT TO LGA ARPT VIA THE MINKS 2 ARR; WE WERE GIVEN AN ALT TO CROSS SOMTO INTXN AT 13000 FT MSL. I WAS USING VERT DSCNT TO DSND THE ACFT VERSUS VNAV DUE TO TURB. OUR DSCNT RATE TURNED OUT INSUFFICIENT TO CROSS THE FIX AT OUR ASSIGNED ALT. WHEN QUESTIONED ABOUT OUR ALT UPON XING SOMTO INTXN; WE REALIZED OUR ERROR. AT THAT TIME I COULDN'T REMEMBER BEING GIVEN THE RESTR FOR SOMTO; BUT JUST OUR CLRNC TO 13000 FT MSL. I TOLD THE CTLR THAT AND HE STATED THAT HE HAD INDEED GIVEN THE SOMTO RESTR. THE ERROR WAS MADE DUE TO A LACK OF MONITORING OUR DSCNT AND THE NEED FOR MORE CONCENTRATED MONITORING IS APPARENT. MY ONLY QUESTION ABOUT THE CTLR'S ACTION WAS THE LATE QUERY ABOUT OUR ALT.