Prolonged Tachypnea

My son is 3 weeks old. He has been experiencing episodic tachypnea since he was born. He spent several nights in NICU for monitoring. All studies came back normal (labs, chest xray, & echo) They dont know why he is still having the tachypnea and are hoping he just outgrows it as his o2 stays good. His primary care doctor said he has never seen a baby have it for more than a couple days so he isnt sure what to think of it. Has anyone here had a baby with prolonged Tachypnea? He will have episodes of 80-120 breath a min. Episodes can last up to 2.5 hours.