A319 CREW AT LGA WAS INITIALLY CLRED TO CLB TO 5000 FT; BUT DURING CLB INSTRUCTED TO LEVEL OFF BECAUSE OF TFC.

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WE WERE NEXT TO DEPART ON RWY 4 AT LGA AND WERE MONITORING THE TWR FREQ. AS AN ACFT WAS LNDG WE OVERHEARD THE TWR TELL ANOTHER ACFT (THAT WAS BEHIND THE ACFT LNDG) THAT THE SPACING WAS INADEQUATE AND TO GO AROUND. THEY WERE INSTRUCTED (TO THE BEST THAT I CAN RECALL) TO MAINTAIN 3000 FT AND FLY A SPECIFIC HDG. I BELIEVE THERE WAS SOME CONFUSION WITH THE ALT BTWN 2000-3000 FT BTWN THE GAR ACFT AND THE CTLR. THE PUBLISHED MISSED APCH ALT IS 2000 FT. AT THIS POINT WE RECEIVED A CALL FROM THE FLT ATTENDANTS CONCERNING THE PAX READING LIGHTS. THE CAPT ADDRESSED THE PROB BY TELLING THEM TO TURN THE SWITCH ON. THE TWR CLRED US INTO POS AND HOLD AND THEN FOR TKOF. PER OUR CLRNC; WE WERE CLRED TO TURN L TO 090 DEGS AND TO MAINTAIN 5000 FT. WE WERE TOLD TO CONTACT DEP. WE CHKED IN WITH DEP AND BEGAN OUR AFTER TKOF DUTIES AND CHKLIST. DEP CALLED US AND TOLD US TO LEVEL OFF. DEP THEN TOLD AN ACFT TO CLB IMMEDIATELY. WE BEGAN OUR LEVELOFF AT ABOUT 2200 FT AND LEVELED AT ABOUT 2500 FT. WE IMMEDIATELY REQUESTED CLARIFICATION REGARDING WHOSE INSTRUCTION IT WAS TO CLB. WE WERE INSTRUCTED TO LEVEL OFF AGAIN. ON TCASII WE SAW AN ACFT 500 FT ABOVE AND SLIGHTLY BEHIND OUR POS. NO TCASII TFC TA OR RA WAS INDICATED. IT IS IMPOSSIBLE FOR US TO TELL HOW CLOSE WE WERE TO THE OTHER ACFT OR TO DETERMINE IF IT WAS A MISS SIT OR NOT. I BELIEVE THE PROB AROSE DUE TO HIGH TFC LOAD FOR THE CTLR; THE SPACING ON THE APCH AND THE GAR OF THE ACFT ON FINAL. CONTRIBUTING FACTORS WERE THE IMC CONDITIONS; THE CONFUSION ABOUT THE GAR/MISSED APCH ALT AND TO A SMALLER EXTENT THE DISTR FROM OUR FLT ATTENDANTS.