ACR Y CLB THROUGH OCCUPIED ALT HAD LTSS FROM MLT. SYS ERROR.

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I WAS WORKING A DEP RUSH ON A LOW ALT SECTOR. THERE WERE 3 OVERFLTS WITH THE MAJORITY OF THE TFC BEING TURBOJETS CLBING FROM THE ARPT. THE TFC COMPLEXITY WAS RAPIDLY INCREASING SO I ASKED FOR ASSISTANCE. THERE WAS A SWBOUND MLT AT FL180. THE TURBOJETS ARE TRANSFERRED TO THE CTR CLBING WBOUND TO 17000 FT. I CLBED SEVERAL ACFT TO FL230 AND SOME WERE STOPPED AT FL200 FOR AN OVERFLT AT FL210. NO TEMPORARY ALTS WERE ENTERED INTO THE DATA BLOCK. I WENT BACK TO AN ACFT THOUGHT TO BE IN CONFLICT WITH THE MLG AND STOPPED HIM AT 17000 FT. ANOTHER JET CHKED ON AND I LEFT HIM AT 17000 FT. I THOUGHT I HAD STOPPED YET ANOTHER ACFT; ACR Y; AT 17000 FT. I WAS EXTREMELY BUSY BY THIS TIME. I ISSUED TFC TO ACR Y 'TFC 2 O'CLOCK; 2 MI; SWBOUND AT FL180.' ACR Y ACKNOWLEDGES 'IN SIGHT.' I THEN ISSUED TFC TO THE MLT; 'TFC 9-10 O'CLOCK; 2 MI; WBOUND; CLBING TO 17000 FT.' THE MLG ACKNOWLEDGES TFC IN SIGHT. THE NEXT THING I NOTICED WAS ACR Y MODE C INDICATING 17600 FT. I ASKED HIM TO VERIFY LEVEL AT 17000 FT AND HE REPLIED; 'NO WE WERE CLRED TO FL230 AND WE HAD THE TFC IN SIGHT.' THE ACFT'S CLOSEST PROX WAS 1 MI AND 100 FT. SEVERAL FACTORS CAN BE LEARNED FROM THIS INCIDENT: 1) THE USE OF TEMPORARY ALTS IN THE DATA BLOCKS. 2) WHEN ISSUING TFC; RESTATE THE ALT TO BE MAINTAINED. 3) STAFF SECTORS SOONER SO YOU CAN HAVE THE EXTRA SCANNING. I AM NOT SURE HOW MANY PLTS ARE AWARE OF SEPARATION STANDARDS FOR VARIOUS FACILITIES. THE PLT OF ACR Y RECEIVED AN RA BUT HE DID NOT RESPOND TO IT BECAUSE HE HAD VISUAL. WE CANNOT USE VISUAL SEPARATION IN THE CTR ENVIRONMENT. BOTH PLTS HEARD THE TA'S. HAD THE PLT OF ACR Y QUESTIONED THE ASSIGNED ALT AFTER HEARING THE TA TO THE MLT THIS SIT MAY NOT HAVE OCCURRED. ANOTHER DEFICIENCY IS THE STAFFING. WE CONTINUALLY WORK SHORT STAFFED. THIS SIT OCCURRED DURING A KNOWN RUSH TIME AND WAS NOT FULLY STAFFED PRIOR TO THE CTLR GETTING VERY BUSY.