FLC OF LR35 HAD LOSS OF SEPARATION THE FIRST TIME; AND AN NMAC THE SECOND TIME WITH 2 UNK ACFT DURING AN IFR CLB. ON EACH OCCASION; ATC HAD CLRED THEM TO AN ALT AT WHICH THERE WAS ALREADY TFC AND AFTER THEY HAD PASSED THROUGH THE LOWER ALT GIVEN IN THEIR AMENDED CLRNC.

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FLYING AN LR35-A WITH 2 VERY EXPERIENCED (9 YRS) LEAR JET CAPTS ON BOARD; WE WERE INVOLVED IN 2 SITS WHICH COULD HAVE LED TO A POSSIBLE MIDAIR COLLISION OR DEFINITELY A NEAR MISS. SIT 1: WHILE ON RADAR VECTORS. SHORTLY AFTER TKOF FROM TEB; WE WERE CLRED TO 7000 FT. THE CO-CAPT (PNF) READ BACK OUR CLRNC TO 7000 FT AND I PROCEEDED TO CLB. (NO ONE CONTESTED OUR CLRNC.) WHEN COMING OUT OF 4500 FT AT ABOUT 230 KTS AND CLBING APPROX 2500 FPM; WE WERE TOLD TO AMEND ALT TO HOLD 4000 FT; THAT WE HAD XING TFC AT 5000 FT. I RESPONDED IMMEDIATELY BY REDUCING PWR AND LOWERING THE ALT TO GO TO 4000 FT. THE CO-CAPT (PNF) IMMEDIATELY TOLD THE CTLR THAT WE WERE ALREADY OUT OF 4500 FT WHEN HE TOLD US TO MAINTAIN 4000 FT; BUT WE WERE MAKING IMMEDIATE CORRECTIONS AND THAT WE HAD THE RPTED TFC IN SIGHT. I ESTIMATE THAT LEGAL SEPARATION MIGHT HAVE BEEN SLIGHTLY COMPROMISED. WHEN THE CTLR HANDED US OFF TO THE NEXT FREQ; HE COMMENDED US BY SAYING; WE DID A GOOD JOB BACK THERE AND HE APPRECIATED OUR HELP. SIT 2: SAME FLT; LESS THAN 10 MINS LATER; STILL ON RADAR VECTORS. THE NEXT CTLR CLRED US TO 7000 FT; THEN TO 10000 FT; THEN TO 17000 FT. THE CO-CAPT (PNF) READ BACK OUR CLRNC TO 17000 FT AND I BEGAN CLBING. (NO ONE CONTESTED OUR CLRNC.) A CLB RESTR WAS THEN ISSUED TO AMEND ALT TO MAINTAIN 13000 FT. THIS CLRNC WAS READ BACK AND 13000 FT WAS MAINTAINED. WE WERE THEN CLRED TO CLB TO 17000 FT. ONCE AGAIN THE CO- CAPT (PNF) READ BACK OUR CLRNC TO 17000 FT AND I BEGAN CLBING. (NO ONE CONTESTED OUR CLRNC.) WHEN WE WERE CLBING OUT OF 13500 FT AT ABOUT 250 KTS AND CLBING APPROX 2500 FPM WHEN THE CTLR SAID; 'YOU WERE TOLD TO MAINTAIN 13000 FT YOU HAVE XING TFC AT 14000 FT.' I RESPONDED IMMEDIATELY BY REDUCING PWR AND IMMEDIATELY LOWERING THE ALT TO GO TO 13000 FT. THE CO-CAPT (PNF) IMMEDIATELY TOLD THE CTLR THAT WE WERE MAKING IMMEDIATE CORRECTIONS TO DSND TO 13000 FT AND THAT WE HAD THE RPTED TFC IN SIGHT. I ESTIMATE 300-500 FT HORIZ SEPARATION AND 300-500 FT VERT SEPARATION. WE PASSED BELOW AND BEHIND THE RPTED ACFT. BOTH SIT 1 AND SIT 2 WERE IN VFR DUSKY; DARK VISIBILITY. VISIBILITY; HOWEVER; WAS PRETTY GOOD FOR THAT CONDITION. TFC IN THE AREA WAS WHAT I WOULD CONSIDER TO BE FAIRLY HVY. THE CTLRS WERE FRIENDLY AND WERE DOING VERY PROFESSIONAL GOOD JOBS BUT THEY WERE FAIRLY BUSY. AS I RECALL THERE WERE OTHER ACFT IN THE AREA WITH SIMILAR CALL SIGNS. I FIRMLY BELIEVE; HAD THE VISIBILITY BEEN MORE REDUCED; THIS SIT COULD HAVE BEEN A LOT MORE SERIOUS THAN IT WAS.