Don,
My wife has the same ailment and has been diagnosed by her cardiologist as having Coronary artery spasm, which can bring on a heart attack and other serious complications. She tested perfect in all test even heart cath, but none the less has been prescribed a Nitro patch which so far has kept those attacks at bay.
This diagnosis has been resisted by about half the cardiologist population because the Nitro patch is the only thing they can do about it so far.
Check it out here:
http://www.mayoclinic.com/health/coronary-artery-spasm/AN01371Bill G