Actually Neuro Cariac Sycope has little to do with blood, ishemia and isn't related to heart attacks and won't show anything on an EKG.
Have you been tested for this? If not see if you can have a tilt table test done. And even though an EKG won't show this specifically one should be done too, might be an arrythmia.
Oh, and the big triggers for this type of syncope are heat, dehydration and alcohol.
What happens is that when the peripheral vasculature relaxes in response to heat whatever the blood pressure drops slightly. Normally the pressure regulation center in the heart will sense this and increase the HR slightly to continue adequate perfusion. In this case the opposite happens and the HR drops. The resulting lack of blood flow can cause dizzyness and fainting.
There are some medications that can help but the key is to stay hydrated, be careful riding in the heat and avoid alcohol.