A B737-300 CREW DID NOT COMPLY WITH A XING RESTR ISSUED BY ATC ON DSCNT INTO HOU.

Narrative

ON FLT FROM ONT TO IAH PRIOR TO DSCNT INTO IAH; I LEFT THE COCKPIT TO USE THE LAVATORY. WHILE IN THE BACK; NOTICED THAT THE ACFT STARTED TO DSND. I RETURNED TO THE COCKPIT AND WHEN I TOOK OVER; ATC GAVE US A XING RESTR AT HOAGI OF 10000 FT AND 250 KTS; WHICH WAS IN THE FMC. HOWEVER; THE CAPT AHD THE ACFT IN A CRUISE DSCNT; SO WHEN I SELECTED A PATH DSCNT THE FMC PROMPTLY INFORMED ME WE WERE 5000 FT HIGH. I ELECTED TO USE THE FGS TO TRY AND CATCH UP. HOWEVER; WITH TAILWINDS IT BECAME OBVIOUS THAT THIS WAS NOT GOING TO WORK. I ASKED THE CAPT TO INFORM AT THAT WE COULD NOT MAKE THE RESTR. HE INSTEAD ASKED ME TO USE LEVEL CHANGE INSTEAD OF VERT SPD. I TRIED THIS AND THE ACFT ENDED UP IN AN OVERSPD SIT AND WE DISENGAGED THE AUTOPLT TO REGAIN CTL. THE CAPT TOLD ATC THAT WE COULD MAKE THE ALT; BUT NOT THE AIRSPD. I INFORMED HIM THAT I DID NOT THINK THAT WAS POSSIBLE; BUT RADIO CONGESTION AND GND SPD DID NOT HELP AND WE ENDED UP HIGH. THERE WAS NO CONFLICT WITH OTHER ACFT AND ATC DID NOT QUESTION THE SIT. I BELIEVE THAT CONTRIBUTING FACTOR WAS THE FACT THAT I LEFT THE COCKPIT AT A CRITICAL TIME AND THAT THE CAPT PUT THE ACFT IN A CRUISE DSCNT INSTEAD OF CAPTURE. AND I DO NOT BELIEVE CRM WORKED IN THIS SIT. LOOKING BACK; IF THE CAPT HAD INFORMED ATC AS REQUESTED AND WE HAD TALKED ABOUT IT LATER BEING THAT TIME WAS CRITICAL AND THIS WOULD HAVE BEEN THE SAFEST AND MOST CONSERVATIVE THING TO DO; A POSSIBLE CONFLICT WOULD HAVE BEEN AVOIDED. SUPPLEMENTAL INFO FROM ACN 497080: FO WAS FLYING WHEN ATC CLRED CREW TO CROSS HOAGI INTXN AT 10000 FT. FMC WENT TO VNAV SPD VERSUS VNAV PATH AND ACFT WAS 2000 FT HIGH AT HOAGI. ATC SAID TO DELETE THE 250 SPD RESTR AND TO TRY TO MAKE HOAGI AT 10000 FT. ALT AT HOAGI WAS APPROX 12000 FT MSL. PNF (CAPT) WAS MAKING IN-RANGE CALL TO COMPANY; PAX ARR PA; AND ACCOMPLISHING IN-RANGE CALL DURING DSCNT AND WAS NOT MONITORING VERT PATH.