Shortness of breath combined with chest pain

Hi! I'm new here and this may not be the right place to post this.  If not, please direct me to the right place.  My husband has been struggling off and on with chest pain especially after strenuous activity, mainly intercourse (sorry TMI) and sometimes after a long run. He is a marathon runner.  In May he passed out after having some dental work done due to a drop in blood pressure and had to be driven home.  Last week, after intercourse, he laid down due to being lightheaded and had shortness of breath and chest pain for about 3 minutes.  

He has been to our regular doctor and he says there is nothing wrong.  I believe something different, my husband does not.  Isn't this a serious thing?  What could be happening?  He's running a marathon and running in very fast to qualify for Boston and I'm worried that this will happen again.  

Background on him:
30
African (not african american like me, just plain african)
8% body fat
6' 170 lbs
very fit and athletic
does not smoke or drink at all
no red meat, extremely heathly
regular vitamins including zinc, folic acid, and one a days mens
father died of "lung cancer" at 46
uncle and granddad died in 40s as well unknown causes due to poor medical attention.

I think that's it.

Thanks for all your help!