GIV GREW WERE INITIALLY ISSUED CLB TO FL280. LATER; ATC AMENDED THE ALT TO FL260; WHICH BOTH CREW MEMBERS STATE THEY HAD MISSED. THE CREW FAILED TO RESET THEIR FGP WHICH WAS SET TO FL280.

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ON ENRTE CLB AFTER DEPARTING PDK; GA; WE WERE CLRED TO FL280 ON A HDG OF APPROX 250 DEGS. A SHORT TIME LATER; WE WERE GIVEN A CLRNC OF DIRECT TO GCV AFTER PASSING FL240. AFTER FL240 WE INITIATED A TURN FOR THE COURSE TO GCV. SOMETIME DURING OUR DISCUSSION ABOUT OUR RTE; WE BOTH MISSED THE CLRNC TO STOP OUR CLB AT FL260. WITH FL280 IN OUR FGP; AS WE APCHED FL270; THE CTLR ASKED OUR ALT. AFTER MY REPLY HE TOLD US TO MAINTAIN FL270. HE ASKED IF WE GOT THE FL260 LEVELOFF. I DON'T REMEMBER HAVING READ BACK SUCH A CLRNC. HOWEVER; THERE HAD BEEN SEVERAL VERT AND LATERAL CLRNCS GIVEN IN A SHORT TIME FRAME. I COULD HAVE MISSED IT BTWN PROGRAMMING OUR FMS AND DISCUSSING OUR RTE. BEFORE SWITCHING; THE CTLR TOLD US THAT HE HAD LOST OUR MODE C (NO SPECIFIC ALT GIVEN) AND WHEN HE REGAINED MODE C WE WERE HEADED FOR FL270. WE HAD NO PREVIOUS MODE C COMPLAINTS. OUR TCASII INDICATED NO TFC TARGETS AND THERE WERE NO TFC CONFLICTS. OUR COMPANY SOPS ARE QUITE EXPLICIT AS TO VERT AND LATERAL POS AWARENESS. THE CREW AND CTLR WERE TRYING TO PERFORM TOO MANY TASKS IN SHORT TIME LINE. I BELIEVE OUR SITUATIONAL AWARENESS SUFFERED AND THIS EVENT WAS THE RESULT. ANOTHER CONTRIBUTING FACTOR THAT RECEIVES LITTLE OR NO CONSIDERATION IS THAT OF LEAVING FROM ARPTS SUCH AS PDK AND BEING HELD UNDER CLASS B AIRSPACE 'LOW AND SLOW' FOR 30 OR 40 MI AND STEP-CLBED AT 2000-3000 FT INCREMENTS WITH MULTIPLE VECTORS TO ACCOMMODATE OTHER TFC ON SIDS. COMPLEX; HIGH PERFORMANCE AND AUTOMATED ACFT SHOULD BE ROUTED ON SIDS ASAP; GET INTO OR ABOVE CLASS B AND ENRTE TO REDUCE EXPOSURE TO THIS TYPE OF EVENT.