ACR HAS LTSS ON CLB OUT.

Narrative

UPON DEPARTING RWY 31 AT LGA; WE WERE ASSIGNED A 340 DEG HDG AND CLRED TO 5000 FT. AFTER TKOF THE TWR INSTRUCTED US TO CONTACT DEP CTL AS WE WERE CLBING THROUGH 1000 FT. WE CHANGED FREQS AS INSTRUCTED BUT WERE UNABLE TO CONTACT DEP DUE TO A STUCK MICROPHONE ON THE FREQ. WE CONTACTED LGA TWR AGAIN AND WERE IMMEDIATELY ASSIGNED ANOTHER FREQ. AT ABOUT THIS TIME WE WERE APCHING 4000 FT AND NOTICED ANOTHER ACFT IN FRONT OF US AND SLIGHTLY ABOVE PASSING L TO R. WE REDUCED OUR RATE OF CLB AND CONTACTED DEP ON THE NEW FREQ. THE CTLR IMMEDIATELY INSTRUCTED US TO STOP OUR CLB AT 4000 FT (WE HAD BEEN CLRED TO 5000 FT); BUT WE OVERSHOT AS WE LEVELED OFF ABRUPTLY; REACHING A MAX ALT OF ABOUT 4200 FT. THE TFC AHEAD OF US WAS ABOUT 1 MI AWAY AT 5000 FT. ALTHOUGH I DO NOT BELIEVE THAT A COLLISION WOULD HAVE OCCURRED; WE PROBABLY WOULD HAVE PASSED UNCOMFORTABLY CLOSE TO THE OTHER ACFT IF WE HAD CLBED ALL THE WAY TO OUR CLRNC ALT. IN THIS CASE BOTH WE AND THE CTLR RECOGNIZED THE POTENTIAL CONFLICT AND WE EACH TOOK CORRECTIVE ACTION. THE STANDARD DEP AT LGA FROM RWY 31 CALLS FOR A CLB TO 5000 FT; SO I CAN ONLY ASSUME THAT SOMEONE HAD ERRONEOUSLY VECTORED THE OTHER ACFT THROUGH THE DEP CORRIDOR AT AN INAPPROPRIATE (AND DANGEROUS) ALT. AS PLTS WE ARE REQUIRED TO ADHERE TO OUR CLRNC RTING AND ALT IN THE EVENT OF COM FAILURE; AND WE THEREFORE DEPEND UPON ATC TO PROVIDE US WITH SAFE PASSAGE. IF WE HAD BEEN IN INST METEOROLOGICAL CONDITIONS AND WERE UNABLE TO ESTABLISH CONTACT WITH ATC; THIS INCIDENT COULD HAVE BEEN MUCH MORE DANGEROUS. THIS SERVES AS ONE MORE EXAMPLE OF THE IMPORTANCE OF ALWAYS VISUALLY SCANNING FOR TFC IN VMC.